PVA Gateway

Quick Start

First install P4P (see the main Quick Start).

The following commands will set up a gateway instance named mygw on a Linux system that uses systemd:

  # generate a simple configuration file
sudo python -m p4p.gw --example-config /etc/pvagw/mygw.conf
  # generate a systemd unit file to support the gateway
sudo python -m p4p.gw --example-systemd \
     /etc/systemd/system/pvagw@.service
  # start the gateway
sudo systemctl daemon-reload
sudo systemctl start pvagw@mygw.service
  # check to see if the instance has started correctly
sudo systemctl status pvagw@mygw.service
  # set the instance to start automatically on boot
sudo systemctl enable pvagw@mygw.service

Background

The PVA Gateway provides a way for EPICS client software to access IOCs on an isolated network.

In doing so, it reduces the resource load on the server-facing side, and provides access control restrictions to requests from the client facing side. The gateway is a specialized proxy for the PV Access (PVA) Protocol which sits between groups of PVA clients and servers. (see What is PVAccess?)

PVA Connections without a Gateway

Without a Gateway, M clients connect to N servers with M*N connections (TCP sockets). If all clients are subscribed to the same set of PVs, then each server is sending the same data values M times.

PVA Connections through a Gateway

Adding a Gateway reduces the number of connections to M+N. With M clients connecting to a gateway server on one side, and one gateway client connecting to N servers on the other. Further, a Gateway de-duplicates subscription data updates so that each server sends only a single update to the Gateway, which then repeats it to each client.

So the PVA servers and IOCs see only a single client, and are shielded from a potentially large number of clients on the other side of the gateway.

Note

Each gateway process can define multiple internal Servers and Clients. This allows, for example, a single gateway process to connect to multiple IOC subnets, providing EPICS clients to access all IOCs.

Example

A common scenario is to have a gateway running on a host computer with two network interfaces (NICs) on different subnets, and thus two different broadcast domains.

In this example, a server has two NICs with IP addresses 192.168.1.5/24 and 10.1.1.4/24.

Example: A Multi-homed Host for a Gateway

To support this host, the gateway can be set up with the following configuration file. The intent is that the gateway provides EPICS clients on the 10.1.1.0/24 subnet with access to IOCs or other PVA servers on the 192.168.1.0/24 subnet.

Each of the statements in this configuration file are explained below

/* C-style comments are supported */
{
    "version":2,
    "clients":[
        {
            "name":"client192",
            "addrlist":"192.168.1.255",
            "autoaddrlist":false
        }
    ],
    "servers":[
        {
            "name":"server10",
            "clients":["client192"],
            "interface":["10.1.1.4"],
            "addrlist":"10.1.1.255",
            "autoaddrlist":false,
            "statusprefix":"GW:STS:" /* optional, but suggested */
        }
    /* optional, allows server side access to Gateway status */
        ,{
            "name":"server192",
            "clients":[],
            "interface":["192.168.1.5"],
            "addrlist":"192.168.1.255",
            "autoaddrlist":false,
            "statusprefix":"GW:STS:"
        }
    ]
}

The version statement is described below.

The clients section specifies the name of its only Client to be client192 and is configured to search on the 192.168.1.0/24 subnet by providing the 192.168.1.255 broadcast address as the only member of the addrlist. This is the network to which an EPICS IOC is attached, so it will receive broadcast searches from this gateway acting as a client.

The servers section specifies the name of its first Server to be server10, and indicates which clients can have access to it, in this case clients which are part of the clients192 section. It is configured to listen on the 10.1.1.0/24 subnet by specifying the local interface address 10.1.1.4. This is the network on which an EPICS client such as pvget or pvput is attached, and this gateway will act as a Server to receive their search messages. The interface broadcast address is also provided to enable sending of server beacon packets. This is an optimization to reduce connection time, and it is not required.

The statusprefix value is set to GW:STS: in this example, allowing the gateway to share some internal PVs which provide status information. The Status PVs suffixes are described below, with the statusprefix prepended. Sites with multiple gateways on one subnet should give each a unique statusprefix.

A second servers section is shown, with its name set to server192. Its set of allowed clients is empty, but interfaces and address lists are specified. This allows the status PVs mentioned above to be accessed from the subnet hosting the IOCs and other EPICS servers. Without this section, those status PVs are only accessible from EPICS clients on the client subnets.

Note

A single gateway will not connect to itself (no Gateway client will connect to a Gateway server in the same instance). However, this automatic loop avoidance is not possible in more complex situations involving multiple gateways. If such a setup is judged necessary, care should be taken to ensure that loops can not form. See also the servers[].ignoreaddr in Configuration File Keywords.

Command Line Arguments

usage: pvagw [-h] [--no-ban-local] [-v] [--logging LOGGING] [--debug] [-T] [--example-config] [--example-systemd]
             config

Positional Arguments

config

Config file

Named Arguments

--no-ban-local

Legacy option. Ignored

Default: False

-v, --verbose

Enable basic logging with DEBUG level

Default: 20

--logging

Use logging config from file (JSON in dictConfig format)

--debug

Enable extremely verbose low level PVA debugging

Default: False

-T, --test-config

Read and validate configuration files, then exit w/o starting a gateway. Also prints the names of all configuration files read.

Default: False

--example-config

Write an example configuration file and exit. “–example-config -” writes to stdout

Default: False

--example-systemd

Write an example systemd unit file and exit “–example-systemd -” writes to stdout

Default: False

Configuration File

Configuration is provided as a file using JSON syntax with C-style comments.

/* C-style comments allowed */
{
    "version":2,
    "readOnly":false,
    "clients":[
        {
            "name":"theclient",
            "provider":"pva",
            "addrlist":"...",
            "autoaddrlist":false,
            "bcastport":5076
        }
    ],
    "servers":[
        {
            "name":"theserver",
            "clients":["theclient"],
            "interface":["..."],
            "addrlist":"",
            "ignoreaddr":["..."],
            "autoaddrlist":false,
            "serverport":5075,
            "bcastport":5076,
            "getholdoff":1.0,
            "statusprefix":"PV:",
            "access":"somefilename.acf",
            "pvlist":"somefilename.pvlist"
        }
    ]
}

See also PVXS client and server configuration references.

Run pvagw --example-config - to see another example configuration.

Configuration File Keywords

Here is a full list of JSON keys available for the configuration file, version 2.

version

JSON Scheme version number. 2 is recommended for new files. Valid values are 1 or 2.

readOnly (default: false)

Boolean flag which, if set, acts as a global access control rule which rejects all PUT or RPC operations. This takes precedence over any ACF file rules.

clients

List of Gateway Client configurations.

clients[].name

Unique name for this Client within this gateway process.

clients[].provider (default: “pva”)

Selects a ChannelProvider. Currently only “pva” is valid.

clients[].addrlist (default: “”)

List of broadcast and unicast addresses to which search messages will be sent.

clients[].autoaddrlist (default: true)

Whether to automatically populate addrlist with all local interface broadcast addresses. Use caution when setting true.

clients[].bcastport (default: 5076)

UDP port to which searches are sent.

servers

List of gateway Server configurations.

servers[].name

Unique name of this Server within this gateway process.

servers[].clients

A list of zero or more gateway Client names. Search requests allowed through this server will be made through all listed clients.

servers[].interface (default: [“0.0.0.0”])

A list of local interface addresses to which this gateway Server will bind.

servers[].addrlist (default: “”)

List of broadcast and unicast addresses to which beacon messages will be sent

servers[].ignoreaddr (default: “”)

List of address to add into the banned list to explicit ignore hosts.

servers[].autoaddrlist (default: true)

Whether to automatically populate addrlist with all local interface broadcast addresses. Use caution when setting true.

servers[].serverport (default: 5075)

Default TCP port to bind. If not possible, a random port will be used.

servers[].bcastport (default: 5076)

UDP port bound to receive search requests, as well as the port to which beacons are sent.

servers[].getholdoff (default: 0)

A value greater than zero enables rate limiting of Get operations. getholdoff defines a hold-off time after a GET on a PV completes, before the another will be issued. Another GET for the same PV made before the hold-off expires will be delayed until expiration. Concurrent GET operations may be combined.

This activity is per PV.

servers[].statusprefix (default: “”)

The text used by this gateway as a prefix to construct names for PVs which communicate status information. The PVs report overall status for the gateway process, regardless of the number of internal Clients or Servers. Each of the status PVs are defined in Status PVs. Note that the prefix will typically end with the delimiter used in your PV naming convention, such as :.

servers[].access (default: “”)

Name an ACF file to use for access control decisions for requests made through this server. See ACF Rules File. Relative file names are interpreted in relation to the directory containing the config file.

servers[].pvlist (default: “”)

Name of PVList file used to restrict access to certain PVs through this Server. See PVList File. Relative file names are interpreted in relation to the directory containing the config file.

servers[].acf_client

Needed only if access key is provided, and clients list has more than one entry. Unambiguously selects which client is used to connect INP PVs for use by conditional ACF rules. If not provided, then the first client in the list is used.

Status PVs

Servers with the statusprefix key set will provide access to the following PVs. These values are aggregated from all defined internal gateway Servers and Clients.

Warning

The PV names resulting from the statusprefix and the PV suffixes shown below must be unique across your site. Each gateway instance must have a unique statusprefix value.

<statusprefix>asTest

An RPC only PV which allows testing of PVList and ACF rules.

$ pvcall <statusprefix>asTest pv=some:name

Other arguments include user="xx", peer="1.1.1.1:12345, and roles=["yy"]. If omitted, the credentials of the requesting client are used.

<statusprefix>clients

A list of client’s names connected to the GW server

<statusprefix>cache

A list of channels to which the GW Client is connected

<statusprefix>refs

Table of object type names and instance counts. May be useful for detecting resource leaks while troubleshooting.

<statusprefix>threads

Available when running with python >= 3.5. An RPC call which returns a text description of all python threads.

Note

The following PVs provide data bandwidth information for the overall gateway.

• The ds in the names refer to downstream requests from EPICS clients to the gateway, or responses from the gateway to EPICS clients.

• The us in the names refer to upstream requests from the gateway to IOCs, or responses from an IOC to the gateway.

• The bypv or byhost in the names refer to status relating to the involved PVs or host machines, respectively.

• The rx and tx in the names refer to receiving or transmitting data from the gateway’s perspective.

<statusprefix>ds:bypv:rx

A table containing bandwidth usage of requests for each PV sent from PVA clients such as pvget or pvput to this gateway. This can be a relatively low number since the requests are often small in size. The table is sorted from highest bandwidth PVs to lowest.

<statusprefix>us:bypv:tx

A table containing bandwidth usage of requests for each PV sent from this gateway to PVA Servers such as IOCs. This can be a relatively low number since the requests are often small in size. The table is sorted from highest bandwidth PVs to lowest.

<statusprefix>us:bypv:rx

A table containing bandwidth usage of responses from each PV sent from PVA Servers such as IOCs to this gateway. The table is sorted from highest bandwidth PVs to lowest.

<statusprefix>ds:bypv:tx

A table containing bandwidth usage of responses from each PV sent from this gateway to EPICS clients that made the original requests. The table is sorted from highest bandwidth PVs to lowest.

<statusprefix>ds:byhost:rx

A table containing bandwidth usage of each host sending requests from PVA clients such as pvget or pvput to this gateway. This can be a relatively low number since the requests are often small in size. The table is sorted by host machine with the highest bandwidth usage to lowest.

<statusprefix>us:byhost:tx

A table containing bandwidth usage of requests sent from this gateway to each host containing PVA Servers such as IOCs. This can be a relatively low number since the requests are often small in size. The table is sorted by host machine with the highest bandwidth usage to lowest.

<statusprefix>us:byhost:rx

A table containing bandwidth usage of each host providing responses from PVA Servers such as IOCs to this gateway. The table is sorted by host machine with the highest bandwidth usage to lowest.

<statusprefix>ds:byhost:tx

A table containing bandwidth usage of each client’s host accepting responses from this gateway. The table is sorted by host machine with the highest bandwidth usage to lowest.

Log File Configuration

The gateway is able to record messages associated with important events to one or more destinations as it runs, including log files or a console device. The messages can be debugging aids for developers, or errors encountered as the gateway is working. It also records the time at which the gateway starts or stops, and when starting, lists the configuration details for the internal clients and servers, and lists each status PV that the gateway will make available.

A python dictConfig logging configuration file in JSON format can be passed to --logging to provide control of formating of the logged messages.

One logger name of special interest is p4p.gw.audit which is used for messages arising from TRAPWRITE and Put logging.

The following is an example of a log configuration file which records INFO messages or worse to a log file, but also records WARNING messages or worse to the computer console. It specifies different formats for console-bound messages versus log file messages, and instructs the system to maintain daily log files (and audit files, if enabled), in a subdirectory called BL3-LOGS. It will create new, empty log files each midnight while keeping previous log files for 14 days.

Note that fixed-width columns are specified for some fields using sequences like 15s, -4d or 4.4s, similar to printf style format specifiers:

{
    "version": 1,
    "disable_existing_loggers": false,
    "formatters": {
        "fileFormat": {
            "format": "%(asctime)s | %(name)15s line %(lineno)-4d [%(levelname)4.4s] %(message)s"
        },
        "consoleFormat": {
            "format": "%(asctime)s | %(name)s: %(levelname)s - %(message)s"
        }
    },
    "handlers": {
        "fileMessages": {
            "level": "INFO",
            "class": "logging.handlers.TimedRotatingFileHandler",
            "formatter": "fileFormat",
            "filename": "BL3-LOGS/gateway-BL3-DMZ.log",
            "when": "midnight",
            "interval": 1,
            "backupCount": 14
        },
        "consoleMessages": {
            "level": "WARNING",
            "class": "logging.StreamHandler",
            "formatter": "consoleFormat",
            "stream": "ext://sys.stdout"
        }
    },
    "loggers": {
        "": {
            "handlers": ["fileMessages","consoleMessages"],
            "level": "INFO",
            "propagate": true
        }
    }
}

Access Control Model

A gateway may apply access control restrictions in addition to any restrictions applied by individual IOCs, or other PVA servers, to which it connects. By default a gateway apply no restrictions. A gateway without a PVList File or ACF Rules File will allow all clients to attempt any operation on any PV.

One or more of the readOnly, access, and/or pvlist configuration file keys enable restrictions within a gateway.

The simplest and most direct restriction is the readOnly configuration file key, which applies to all logical Servers within a gateway. If set, no PUT or RPC operations are allowed. Both MONITOR and GET operations are allowed, so readOnly applies a simple one-way policy to allow clients to receive data without being permitted to change any PV settings.

A more granular policy is often desired, which can be expressed with a PVList File and/or ACF Rules File.

Access decisions are made as follows:

1. PV name and client IP address are looked up in a PVList. If DENY, then client searches are ignored.

2. If readOnly is set, then any PUT or RPC operation is rejected. GET/MONITOR proceed.

3. The PV name and peer IP address are looked up in a PVList. If DENY, then the operation is rejected. If ALLOW/ALIAS then an ASG name and level (0 or 1) is found.

4. The ASG name and level are look up in a ACF file. GET/MONITOR operations are always allowed (no write only PVs). PUT or RPC operations are allowed if appropriate WRITE/PUT/RPC permission is granted.

PVList File

The purpose of the PVList file is to specify which PVs are allowed or denied, and to associate those PVs with access security groups (ASG) and security levels (ASL) in the access file. Supported PVList file syntax is mostly compatible with that of the Channel Access Gateway.

If not provided, the default PVList file used is .* ALLOW.

While allows all PV names from all clients.

PVList files are line based, with lines consisting of the following.

Note

Matching is not in strict lexical order. See below.

1. Blank lines and # comments are allowed.

   # comment line
   

2. Evaluation order statement, primarily to maintain backward compatibility with CA Gateway.

   # (default if omitted)
   EVALUATION ORDER ALLOW, DENY
   
   # Not currently supported
   #EVALUATION ORDER DENY, ALLOW
   

3. A DENY statement which specifies that certain PVs are denied access from certain EPICS clients. It can specify an optional host from which clients will be denied access. <PV name regexp> is a regular expression to match PV names. This statement is of the forms

   <PV name regexp> DENY
   <PV name regexp> DENY FROM <hostname_or_IP>
   

1. An ALLOW statement which specifies that certain PVs are allowed to be accessed from EPICS clients. It can specify an optional Access Security Group (ASG), with an accompanying but optional Access Security Level (0 or 1), both of which used when evaluating an ACF file. This statement is of the forms

   <PV name regexp> ALLOW
   <PV name regexp> ALLOW <ASG>
   <PV name regexp> ALLOW <ASG> <ASL_0_or_1>
   

If not provided, ASG is DEFAULT, and ASL is 0.

1. An Alias statement which provides a way to specify a specific PV name based on a more general pattern. This is equivalent to a ALLOW statement with an additional name translation. This statement is of the forms

   <PV name regexp> ALIAS <real PV name>
   <PV name regexp> ALIAS <real PV name> <ASG>
   <PV name regexp> ALIAS <real PV name> <ASG> <ASL_0_or_1>
   

When a gateway Server receives a request from a client to access a PV, the PV name is compared to each pattern in the list.

The order in which regular expressions are matched is that all DENY statements are considered before any ALLOW/ALIAS statements (regardless of the order of lines). PV names which do not match any statement are DENYed.

When a PV name matches more than one ALLOW/ALIAS statements, the order of lines is used. The last match will have effect.

Considering the following PVList file:

ACCL:CRYO:.* DENY
ACCL:.*      ALLOW MISCONFIG
ACCL:.*      ALLOW
ACCL:RF.*    ALLOW RF

ACCL:CRYO:ESTOP would match the DENY rule, so a gateway will not allow any access.

ACCL:RF:FPWR would match the ALLOW RF rule, and be allowed subject to rules for ASG(RF).

ACCL:ARC:CNT would match the last ALLOW rule, and be allowed subject to ASG(DEFAULT).

Because both the ALLOW MISCONFIG and ALLOW rules have identical patterns, the ALLOW will always be used and ALLOW MISCONFIG will never be used.

Note that because DENY rules are always considered before ALLOW or ALIAS rules, the preceding file is functionally identical to the following as moving the DENY relative to ALLOW does not change the evaluation order.

ACCL:.*      ALLOW MISCONFIG
ACCL:.*      ALLOW
ACCL:RF.*    ALLOW RF
ACCL:CRYO:.* DENY

When building a PVList file containing ALLOW or ALIAS rules with overlapping patterns, it is therefore necessary to put the more general patterns before the more specific patterns. eg.

ACCL:RF.*    ALLOW RF
ACCL:.*      ALLOW

In this example the ALLOW RF rule is effectively hidden, and will never be matched.

ACF Rules File

An Access Security File (ACF) is a list of access control rules to be applied to requests based on which Access Security Group (ASG) was selected by a PVList file, or DEFAULT if no PVList file is used. The ASG name selects which a group of rules.

Unknown ASG names use the DEFAULT rules. If no DEFAULT group is defined, then no privileges are granted.

Each ACF file may define zero or more Host Access Groups (HAG s) and/or User Access Groups (UAG s). Also, one or more list of rules (ASG s). The HAG is a list of host names, and the UAG a list of user names.

eg. PVs in ASG DEFAULT only permit PUT or RPC requests originating from hosts incontrol or physics. PUT requests from physics will be logged.

HAG(MCF) { "incontrol" }
HAG(OTHER) { "physics" }
ASG(DEFAULT) {
    RULE(1, WRITE) {
        HAG(MCF)
    }
    RULE(1, WRITE, TRAPWRITE) {
        HAG(OTHER)
    }
}

Privileges

RULE s may grant one of the following privileges.

WRITE

Shorthand to grant both PUT and RPC requests.

PUT

Allow PUT operation on all fields.

RPC

Allow RPC operations.

UNCACHED

Special privilege which allows a client to bypass deduplication/sharing of subscription data. A client would make use of this privilege by including a pvRequest option record._options.cache with a boolean false value.

READ

Accepted for compatibility. PVA Gateway always allows read access for any PV which is allowed by the PVList file. Use a DENY in a PVList file to prevent clients from reading or subscribing to certain PVs.

HAG Hostnames and IPs

Entries in a HAG() may be either host names, or numeric IP addresses. Host names are resolved once on Gateway startup. Therefore, changes in the hostname to IP mapping will not be visible until a Gateway is restarted.

UAG and Credentials

PV Access protocol provides a weakly authenticated means of identification based on a remotely provided user name. This is combined with a set of “role”s taken by looking up system groups of which the username is a member. (See /etc/nsswitch.conf on Linux).

Both user and role names may appear in UAG lists. eg.

UAG(SPECIAL)
{
    root,
    "role/admin"
}

And a rule:

ASG(DEFAULT) {
    RULE(1, WRITE) {
        UAG(SPECIAL)
    }
}

In this case, the RULE will be match if a client identifies itself with username root or if the (remotely provided) username is a member of the (locally tested) admin role (eg. unix group).

In this case, such a match will grant the WRITE privilege for PVs in the DEFAULT ASG.

Role/group membership can be tested with the <statusprefix>asTest status PV.

TRAPWRITE and Put logging

If a RULE includes the TRAPWRITE modifier, then a PUT operation it allows will be logged. Refer to the Log File Configuration section for more information.

Messages are logged through the p4p.gw.audit python logger.

ACF Syntax

acf       ::=  | item acf
item      ::=  uag | hag | asg
uag       ::=  UAG ( "NAME" ) { users }
hag       ::=  HAG ( "NAME" ) { hosts }
asg       ::=  ASG ( "NAME" ) { asitems }
users     ::=  "HOSTNAME"
              "HOSTNAME" , users
hosts     ::=  "USERNAME"
               "USERNAME" , hosts
asitems   ::=  | asitem asitems
asitem    ::=  INP[A-Z] ( "PVNAME" )
               RULE ( ASL#, priv) rule_cond
               RULE ( ASL#, priv, trap) rule_cond
priv      ::=  READ | WRITE | PUT | RPC | UNCACHED
trap      ::=  TRAPWRITE | NOTRAPWRITE
rule_cond ::=  | { conds }
conds     ::=  | cond conds
cond      ::=  UAG ( "NAME" )
               HAG ( "NAME" )
               CALC ( "EXPR" )

Application Notes

The process of configuring a Gateway begins by looking at the physical and/or logical topology of the networks in question.

A Gateway is typically placed at the boundary between one or more networks (subnets).

While a simple Gateway configuration will have a single GW Server connected to a single GW Client, more complicated configurations are possible, with many GW Servers and one GW Client, or one GW Server and many GW Clients, or a many to many configuration.

It is valid for a GW Client and GW Server to be associated with the same host interface and port provided that they are not associated with each other. Pairs of such GW Client and GW Server may be cross linked to form a bi-directional Gateway.

It is meaningful to configure a GW Server with no GW Clients ( "clients":[] ) provided that the "statusprefix" key is set. This server will only provide the status PVs. This can be used to eg. provide GW status monitoring from both sides of a one-way Gateway.

Differences from CA gateway

Summary of known differences from CA gateway.

• EVALUATION ORDER DENY, ALLOW is not supported.

• Permission READ is implied. Write-only PVs are not possible.

Implementation Details

Gateway is implemented as a hybrid of Python and C++. In the interest of performance, Python code is only in the “slow” path for the PV search/connection decision. After a PV is connected; permissions changes, auditing, and monitoring are communicated externally from Python code.

The APIs described below are not currently considered stable or public for use by external modules. They are documentation here for the purposes of internal development and debugging.

Negative Results Cache

In order to shield the Python testChannel() handler from repeated reconnect attempts for denied PVs, a list of blocked PVs, IPs, and pairs of PV and IP is maintained in C++ code. Search requests matching one of these three criteria will be ignored without calling testChannel().

p4p.gw Frontend

This module utilizes the related C++ extension to setup and manage a Gateway which is configured in a manner similar to the pva2pva gateway with an access control policy defined in a manner similar to cagateway. Other means of configuration and policy definition could be implemented.

C++ Extension

Setup execution flow for use of the C++ extension is:

1. Create a ClientProvider

2. Create a Provider using this client

3. Create a p4p.server.Server referencing the provider name.

More than one Provider may reference to the same ClientProvider. A p4p.server.Server may reference more than one Provider, and a Provider may be referenced by more than one p4p.server.Server. Many p4p.server.Server s may be created.

After server startup, the handler object associated with a Provider will be called according to the _gw.ProviderHandler interface.

The C++ extension deals only with IP addresses in string form, possibly with port number (eg. “1.2.3.4:5076)”, and never host names.

class p4p._gw.Provider

Claim

Ignore

BanHost

BanPV

BanHostPV

testChannel(usname)

Add the upstream name to the channel cache and begin trying to connect. Returns Claim if the channel is connected, and Ignore if it is not.

Parameters:

usname (bytes) – Upstream (Server side) PV name

Returns:

Claim or Ignore

sweep()

Call periodically to remove unused Channel from channel cache.

forceBan()

Preemptively Add an entry to the negative result cache. Either host or usname must be not None

Parameters:

• host (bytes) – None or a host name

• usname (bytes) – None or a upstream (Server side) PV name

clearBan()

Clear the negative results cache

cachePeek()

Returns PV names in channel cache

Returns:

a set of strings

stats()

Return statistics of various internal caches

Return type:

dict

report()

Run Client/Upstream bandwidth usage report

Returns:

List of tuple

Return type:

[(usname, opTx, opRx, peer, trTx, trRx)]

class p4p._gw.InfoBase

account

peer

roles

class p4p._gw.CreateOp

Handle for in-progress Channel creation request

create()

Create a Channel with a given upstream (server-side) name

Parameters:

name (bytes) – Upstream name to use. This is what the GW Client will search for.

Returns:

A Channel

name

class p4p._gw.Channel

access()

expired

name

Interfaces

class p4p._gw.ProviderHandler

A Handler object associated with a Provider should implement these methods

testChannel(self, pvname, peer)

Parameters:

• pvname (str) – PV name being searched (downstream)

• peer (str) – IP address of client which is searching

Returns:

Claim, Ignore, BanHost, BanPV, or BanHostPV

Hook into search phase. Called each time a client searches for a pvname. If permitted, call and return the result of Provider.testChannel() with the desired upstream (server-side PV name).

• Returning Claim may result in a later call to makeChannel().

• Returning Ignore may result in a repeated call to testChannel() in future.

• Returning BanHost adds this host to the negative results cache

• Returning BanPV adds this PV to the negative results cache.

• Returning BanHostPV adds this combination of host and PV to the negative results cache

makeChannel(self, op)

Hook info channel creation phase. If permitted, call and return the result of CreateOp.create(). The Channel object may be stored by python code to track and effect active connections. eg. call Channel.access() to set/change privileges. Or Channel.close() to force disconnection.

Due to the continuous nature of PVA client (re)connection process, inability to create a channel at this stage is treated as a hard failure to avoid a reset loop. If it is necessary to return None, then steps should be taken to ensure that a re-connection attempt would have a different result. eg. through Provider.forceBan().

Parameters:

op (CreateOp) – Handle for ongoing operation

Returns:

A Channel.

audit(self, msg)

Hook info PUT logging process. Called from a worker thread.

Parameters:

msg (str) – Message string to be logged