retcl: Tcl client library for Redis

The retcl module is an event-driven, object-oriented, Redis client library for the Tcl programming language. The library exposes a single retcl class, conveniently packaged as a sourceable Tcl module. Instances of this class represent connections to a Redis server and are used to send requests in the form of native Redis commands and retrieve responses.

Other than a few book-keeping methods, retcl instances transparently handle Redis commands as first-class methods. As an example, r SET K Hello can be used to set the value of the key K to the string Hello. This is achieved by proxying all unknown methods to the Redis server by concatenating all arguments, effectively making retcl instances completely decoupled from any version of Redis. This has several advantages:

• A retcl instance does not need to know about the semantics of a particular Redis command. This includes syntax checks, context verification and arguments validation, which are offloaded to the Redis server. As a consequence, the code base remains clean and small.

• New commands introduced by a server upgrade are immediately available to a live application.

package require retcl
retcl create r
r SET key val
r -sync GET key ;# val

Construction

set r [retcl new ?host port? ?option ...?]
set r [retcl new ?-noconnect? ?option ...?]
retcl create r ?host port? ?option ...?
retcl create r ?-noconnect? ?option ...?

Create an instance r of retcl. If no host or port is specified, the client automatically connects to localhost on port 6379. If -noconnect is specified, the client is created in disconnected mode. Each additional option is a simple string or a list representing the invocation of a configuration method. Here are some examples.

retcl create r localhost 6973 -async {+tls -cafile /path/to/ca.crt}
retcl create r -keepCache
retcl create r -noconnect -keepCache +async

Connection / disconnection

$r connect ?host? ?port?
$r disconnect
$r connected

The connect method can be used to connect to a different host and port. It is an error to call this method on an already connected client. The disconnect method can be called no matter the connection status; it disconnects the client from the current host, if any. The connected method can be used to query the current connection status. It returns a true result if the client is connected and a false result otherwise.

Interaction with Redis

set rid1 [$r SET key val] ;# rid stands for result id
$r result $rid1 ;# OK

set rid2 [$r GET key]
$r result $rid2 ;# val

$r resultType $rid1 ;# SimpleString
$r resultType $rid2 ;# BulkString

$r -sync GET key ;# val

proc mycb {id type body} {
    puts "  id: $id"
    puts "type: $type"
    puts "body: $body"
}

$r -cb mycb GET key ;# returns immediately and arrange for mycb to be invoked
                    ;# with {rds:1 BulkString val} when the result arrives

As shown in the examples above, the interaction with Redis is very straightforwards. Any methods not directly understood by the retcl class are forwarded to the Redis server, along with any additional arguments provided. The result is a small string representing a result id. Each call to Redis produces a new result id, which can then be queried to inspect its status, type, and value.

By using the -sync switch, it is possible to have Redis commands block and only return as soon as the result is available. In this case, the return value is the value returned by Redis.

By using the -cb switch, it is possible to arrange for a callback procedure to be called whenever the result is ready. In this case, the command returns immediately.

Configuration

By default, retcl objects operate in asynchronous mode: they return immediately and produce a result id (rid) that can be inspected later on. The methods -async, +async and ?async can be used to disable, enbale, and query this setting. When the asynchronous behaviour is off, methods wait and return the values returned by Redis instead of a result id.

A cache of all results is kept by default. This allows to query previously returned results. The -keepcache, +keepcache, and ?keepcache methods can be used to disable, enable, and query this setting. When the results cache is disabled, results are removed from the cache as soon as they are retrieved by the client.

Error handling

A custom error handler can be setup with the errorHandler method. The argument is a command prefix that gets expanded and additioned with an error message string. Passing an empty command prefix resets the error handler to the default error proc.

Pipelining

A pipeline can be built with the pipeline method. The argument is a script which gets evaluated in the context of the caller. Commands to the Redis server are held for the duration of the script and released as a bulk when the script ends.

Publish / subscribe

Publish / subscribe callbacks for specific items can be specified with the callback method. The item argument is a pattern or channel as in PSUBSCRIBE and SUBSCRIBE. The callback argument is a command prefix. Whenever a message arrives on the specific channel, the command prefix is called by appending the type of the message, the pattern that was subscribed to, the actual channel, and the payload.

TLS

The connection to the Redis server is unencrypted by default. If the TclTLS extension is available, the +tls method can be used to enable TLS. The method takes an optional list of arguments that are passed as-is to the [tls::socket] command. In the default configuration, Redis requires a valid client certificate on connection, which requires specifying a few parameters, e.g., r +tls -cafile /etc/redis/ca.crt -certfile /etc/redis/redis.crt -keyfile /etc/redis/redis.key. The -tls and ?tls methods can be used to disable and query TLS mode. The ping-pong example can be run in TLS mode via tclsh ping-pong.tcl --tls.

Encoding

As per the Redis Serialization Protocol (RESP), commands are sent over the wire as arrays of bulk strings. Bulk strings contain binary data, so all strings need to be converted to byte streams with values in the range 0-255. Unicode strings need to be encoded in utf-8. See the utf8-1.1 test for an example and ticket 70c08b5b5dfor for details.