# 18. Transports

In order to send and receive messages, an AMPS server must allow incoming connections. Transports configure incoming connections to AMPS. Transports are configured in the `Transports` element of the AMPS configuration file.

AMPS provides two distinct kinds of incoming connections:

- Client connections, for use by the AMPS clients to support external applications
- Replication connections, to receive replicated messages from other AMPS instances

Each transport controls how authentication and entitlements are enforced for that transport. The transport can either accept the defaults for the instance as a whole, or choose settings unique to that transport.

## Client Connections

To accept connections from publishers or subscribers, an AMPS instance must have at least one `Transport` configured for client connections. The transport must specify:

- The network protocol used for the transport, called the transport type
- The network address, such as IP address and port, that the AMPS server will listen to for incoming connections
- Optionally, the AMPS command header format, called the protocol. The `amps` protocol is the default if no protocol is provided and is the protocol used by most applications. Websocket connections use the `websocket` protocol.

A transport can optionally set other parameters on the transport. This includes setting the authentication and entitlements that apply to connections for this transport, setting slow client parameters for the transport, and so forth.

### TCP Connections

This is the most commonly used connection type for AMPS clients.

With this option, communication occurs over a standard TCP/IP connection.

### SSL Connections

AMPS supports SSL connections between clients and servers. To enable SSL on a transport, you must:

- Specify a Transport type of `tcp` or `tcps`, and
- Provide a certificate and private key for the connection

You can optionally set other parameters for SSL connections, as described in the AMPS Configuration Reference.

Caution

60East recommends using the `tcps` transport type for SSL connections for clarity. However, AMPS uses SSL connections for a `tcp` connection whenever a PrivateKey and Certificate are provided for a Transport, regardless of whether the transport `Type` is specified as `tcp` or `tcps`.

AMPS clients require that the connection string use `tcps` for SSL connections, even if the AMPS Transport configuration uses `tcp`.

### IPv6 Connections

Starting with version 5.3.3, AMPS supports connections over both IPv4 and IPv6 IP protocols if the host has IPv6 support enabled.

Both IPv4 and IPv6 IP address formats are fully supported for use with specifying the network address of a transport. The IP protocol version used for a transport is determined by the network address format specified for the transport. If only a port is specified and the host supports IPv6, AMPS will listen for incoming connections over both IPv4 and IPv6 protocols.

For outgoing connections, such as Replication Connections, AMPS fully supports both IPv4 and IPv6 IP address formats. AMPS will prefer to resolve a URI to IPv4 addresses by default. The DNS resolution behavior can be specified as part of the configuration of the Replication Destination.

Specific parameters and behavior related to IPv4 vs IPv6 connections are described in the AMPS Configuration Reference.

### Unix Domain Sockets

AMPS provides transports that use unix domain sockets for applications that run on the same system as the AMPS server and require extremely low-latency messaging. Unix domain sockets are not supported by all AMPS clients, since some programming environments do not support these sockets.

With this transport type, many of the configuration settings that apply to TCP/IP sockets are not relevant. Instead, the transport requires the name of a file on the local filesystem as the location at which to create the socket.

### WebSocket Protocol Connections

To allow AMPS to be used directly from within browser-based applications, the AMPS server supports communication over WebSockets. An AMPS client `Transport` that uses `tcp` or `tcps` as the underlying `Type` can be configured to use the websocket `Protocol` for communication. The AMPS JavaScript client library supports `websocket` protocol connections to AMPS (while the other client libraries support `amps` protocol connections).

When a transport is configured to use the `websocket` protocol, a client that connects to that transport sends an HTTP upgrade header indicating that further communication should use the WebSocket protocol. The AMPS server returns a response indicating that it is switching protocols to WebSocket, and further communication between the client and server uses the proprietary AMPS protocol exchanged within WebSocket frames. The contents of the frames are AMPS messages, as described in the AMPS Command Reference. There is no difference in capability between connections that use the `websocket` protocol and connections that use the AMPS protocol. The commands exchanged between the AMPS server and client are the same, although the “on wire” format within which the commands are exchanged is different.

A WebSocket connection to AMPS is a persistent, full-duplex connection to the server. A WebSocket connection is not a RESTful interface to AMPS, nor does the connection support HTTP beyond the initial upgrade request.

## Replication Connections

To receive replicated messages from other AMPS instances, an AMPS instance must have a transport configured as Type `amps-replication`.

Replication connections accept any message type, and can service multiple upstream AMPS instances.

Replication connections are configured as part of an overall High Availability plan. See High Availability and the AMPS Configuration Reference for details.

## Transport Filters

AMPS provides the ability for incoming commands to be modified, or filtered.

When one or more transport filters are specified, AMPS provides each incoming command to those filters as soon as the header for the message is parsed. Each filter can modify the message data or a subset of the headers, and can choose to have AMPS stop processing the command (or can request that AMPS disconnect the connection that submitted the command).

The filters for a `Transport`, if any, are defined in the configuration for the transport. When more than one filter is specified, AMPS runs each filter in the order in which they appear in the configuration file.

Transport filters are implemented as extension modules. To create an extension module, contact AMPS support for the server SDK.

AMPS loads the following transport filters by default:

Module | Description
--- | ---
`amps-topic-translator` | Translates topic names on incoming commands. This module requires one or more of the following options: `Topic` Specifies the translation to use. This option takes the following format: original `:` translated. The original parameter can be a literal topic name or a PCRE regex. Any topic on any command that matches that parameter will be converted to the translated topic. For example, to convert the topic `legacy` to the topic `new`, you would specify the following option:
    <Options>
      <Topic>legacy:new</Topic>
    </Options>
To translate any topic beginning with `/orders/northamerica` to `NAorders`, you would specify the following option:
    <Options>
      <Topic>^/orders/northamerica:NAorders</Topic>
    </Options>
`amps-conflated-topic-translator` | Translates an incoming `subscribe`, `sow_and_subscribe`, `delta_subscribe`, or `sow_and_delta_subscribe` command for a specific topic name as follows: Translates the topic name on the command to a different topic name. Adds a conflation interval to the command, if there is no conflation interval specified on the incoming command. This module can be useful for removing a conflated topic that is infrequently-used, or for which subscribers only monitor a small number of messages out of the overall topic. This module requires one or more of the following options: `Topic` Specifies the translation to use. This option takes the following format: original `:` translated `:` interval The interval specifies the conflation interval to apply to the translated commands if one is not provided. For example, to convert all subscriptions to the topic `orders-C` to the topic `orders`, and guarantee that each translated subscription has a conflation specified, with a 500 millisecond default for conflation, you would use the following options:
    <Options>
      <Topic>orders-C:orders:500ms</Topic>
    </Options>
To convert all subscriptions to the topics `slowUpdates` and `verySlowUpdates` to the topic `updates` and guarantee that each translated subscription has a conflation specified, with a 2 second default for conflation, you would use the following options:
    <Options>
      <Topic>slowUpdates:updates:2s</Topic>
      <Topic>verySlowUpdates:updates:2s</Topic>
    </Options>

Table 18.1: Transport Filters