Redis -- a SWI-Prolog client for redis
Jan Wielemaker
SWI-Prolog Solutions b.v.
The Netherlands
E-mail: jan@swi-prolog.org
Abstract
This package provides the library `redis.pl` that implements a client for the [redis](https://redis.io/), ” An open source (BSD licensed), in-memory data structure store, used as a database, cache and message broker''

# Table of Contents

## 1 About the SWI-Prolog Redis client

Redis is an in-memory key-value store. Redis can be operated as a simple store for managing (notably) volatile persistent data. Redis can operate in serveral modes, ranging from a single server to clusters organised in several different ways to provide high availability, resilience and replicating the data close to the involved servers. In addition to being a key-value store, Redis enables additional communication between clients such as publish/subscribe to message channels, streams, etc.

These features can be used to connect micro services, both for sharing state, notifications and distributing tasks.

### 1.1 Redis and threads

The connection between the redis client and server uses a stream pair. Although SWI-Prolog stream I/O is thread-safe, having multiple threads using this same connection will mixup writes and their replies.

At the moment, the following locking is in place.

- Connections created using redis_connect/3 are not locked. This implies the connection handle may be used from a single thread only, or redis/3 requests must be protected using with_mutex/2.
- Redis/3 request using a server name established using redis_server/3 are locked using a mutex with the same name as the server name.

### 1.2 About versions

The current stable version of Redis is 6. Many Linux distros still ship with version 5. Both talk protocol version 2. Version 6 also supports protocol version 3. The main differences are:

- The version 3 protocol has several improvements that notably improvement passing large objects using a streaming protocol.
- Hashes (maps) in the version 3 protocol are exchanged as lists of pairs (`Name-Value`), while version 2 exchanges hashes as a list of alternating names and values. This is visible to the user. High level predicates such as redis_get_hash/3 deal with both representations.
- The version 3 protocol supports push messages to deal with monitor and subscribe events on the same connection as used for handling normal requests.

New projects are encouraged to use Redis version 6 with the version 3 protocol. See redis_server/3.

### 1.3 Redis as a message brokering system

Starting with Redis 5, redis supports streams. A stream is a list of messages. Streams can be used as a reliable alternative to the older Redis PUB/SUB (Publish Subscribe) mechanism that has no memory, i.e., if a node is down when a message arrives the message is missed. In addition, they can be used to have each message processed by a consumer that belongs to a consumer group. Both facilities are supported by `library(redis_streams)` (section 3)

Redis streams provide all the low-level primitives to realise message brokering. Putting it all together is non-trivial though. Notably:

- We must take care of messages that have been sent to some consumer but the consumer fails to process the message and (thus) ACK it is processed. This is handled by xlisten_group/5 using several options. Good defaults for these options are hard to give as it depends on the required processing time for a message, how common failures are and an acceptable delay time in case of a failure, what to do in case of a persistent failure, etc.
- Streams are independent from consumer groups and acknowledged messages remain in the stream. xstream_set/3 can be used to limit the length of the stream, discarding the oldest messages. However, it is hard to give a sensible default. The required queue length depends on the the size of the messages, whether messages come in more or less randomly or in bursts (that cause the stream to grow for a while), available memory, how bad it is if some messages get lost, etc.

The directory `doc/packages/examples/redis` in the installation provides an example using streams and consumer groups to realise one or more clients connected to one or more compute nodes.

### 1.4 History

This module is based on the `gpredis.pl` by Sean Charles for GNU-Prolog. This file greatly helped me understanding what had to be done, although, eventually, not much of the original interface is left. The main difference to the original client are:

- Replies are not wrapped by type in a compound term.
- String replies use the SWI-Prolog string type.
- Values can be specified as `Value as prolog`, after which they are returns as a (copy of) Value. This prefixes the value using "`\`u0000T`\`u0000".
- Strings are in UTF-8 encoding to support full Unicode.
- Using redis_server/3, actual connections are established lazily and when a connection is lost it is automatically restarted.
- This library allows for using the Redis publish/subscribe interface. Messages are propagated using broadcast/1.

## 2 library(redis): Redis client

This library is a client to Redis, a popular key value store to deal with caching and communication between micro services.

In the typical use case we register the details of one or more Redis servers using redis_server/3. Subsequenly, redis/2-3 is used to issue commands on the server. For example:

    ?- redis_server(default, redis:6379, [password("secret")]).
    ?- redis(default, set(user, "Bob")).
    ?- redis(default, get(user), User).
    User = "Bob"

[det]redis_server(+ServerName, +Address, +Options)
  Register a redis server without connecting to it. The ServerName acts as a lazy connection alias. Initially the ServerName `default` points at `localhost:6379` with no connect options. The `default` server is used for redis/1 and redis/2 and may be changed using this predicate. Options are described with redis_connect/3.

  Connections established this way are by default automatically reconnected if the connection is lost for some reason unless a `reconnect(false)` option is specified.

[det]redis_connect(-Connection)
[det]redis_connect(+Address, -Connection, +Options)
[det]redis_connect(-Connection, +Host, +Port)
  Connect to a redis server. The main mode is `redis_connect(+Address, -Connection, +Options)`. redis_connect/1 is equivalent to `redis_connect(localhost:6379, Connection, [])`. Options:
  reconnect(+Boolean)
    If `true`, try to reconnect to the service when the connection seems lost. Default is `true` for connections specified using redis_server/3 and `false` for explictly opened connections.
  user(+User)
    If `version(3)` and `password(Password)` are specified, these are used to authenticate using the HELLO command.
  password(+Password)
    Authenticate using Password
  version(+Version)
    Specify the connection protocol version. Initially this is version 2. Redis 6 also supports version 3. When specified as `3`, the HELLO command is used to upgrade the protocol.

  Instead of using these predicates, redis/2 and redis/3 are normally used with a server name argument registered using redis_server/3. These predicates are meant for creating a temporary paralel connection or using a connection with a blocking call.

  Address | is a term Host:Port, `unix(File)` or the name of a server registered using redis_server/3. The latter realises a new connection that is typically used for blocking redis commands such as listening for published messages, waiting on a list or stream.

  Compatibility
    `redis_connect(-Connection, +Host, +Port)` provides compatibility to the original GNU-Prolog interface and is equivalent to `redis_connect(Host:Port, Connection, [])`.
[det]redis_disconnect(+Connection)
[det]redis_disconnect(+Connection, +Options)
  Disconnect from a redis server. The second form takes one option, similar to close/2:
  force(Force)
    When `true` (default `false`), do not raise any errors if Connection does not exist or closing the connection raises a network or I/O related exception. This version is used internally if a connection is in a broken state, either due to a protocol error or a network issue.
[semidet]redis(+Connection, +Request)
  This predicate is overloaded to handle two types of requests. First, it is a shorthand for `redis(Connection, Command, _)` and second, it can be used to exploit Redis pipelines and transactions. The second form is acticated if Request is a list. In that case, each element of the list is either a term `Command -> Reply` or a simple Command. Semantically this represents a sequence of redis/3 and redis/2 calls. It differs in the following aspects:

  - All commands are sent in one batch, after which all replies are read. This reduces the number of round trips and typically greatly improves performance.
  - If the first command is `multi` and the last `exec`, the commands are executed as a Redis transaction, i.e., they are executed atomically.
  - If one of the commands returns an error, the subsequent commands are still executed.
  - You can not use variables from commands earlier in the list for commands later in the list as a result of the above execution order.

  Procedurally, the process takes the following steps:

  1. Send all commands
  2. Read all replies and push messages
  3. Handle all callbacks from push messages
  4. Check whether one of the replies is an error. If so, raise this error (subsequent errors are lost)
  5. Bind all replies for the `Command -> Reply` terms.

  Examples

      ?- redis(default,
               [ lpush(li,1),
                 lpush(li,2),
                 lrange(li,0,-1) -> List
               ]).
      List = ["2", "1"].

[semidet]redis(+Connection, +Command, -Reply)
  Execute a redis Command on Connnection. Next, bind Reply to the returned result. Command is a callable term whose functor is the name of the Redis command and whose arguments are translated to Redis arguments according to the rules below. Note that all text is always represented using UTF-8 encoding.

  - Atomic values are emitted verbatim
  - A term A:B:... where all arguments are either atoms, strings or integers (no floats) is translated into a string `"A:B:..."`. This is a common shorthand for representing Redis keys.
  - A term Term as prolog is emitted as "`\`u0000T`\`u0000" followed by Term in canonical form.
  - Any other term is emitted as write/1.

  Reply is either a plain term (often a variable) or a term `Value as Type`. In the latter form, Type dictates how the Redis bulk reply is translated to Prolog. The default equals to `auto`, i.e., as a number of the content satisfies the Prolog number syntax and as an atom otherwise.

  - `status(Atom)` Returned if the server replies with `+ Status`. Atom is the textual value of Status converted to lower case, e.g., `status(ok)` or `status(pong)`.
  - `nil` This atom is returned for a NIL/NULL value. Note that if the reply is only `nil`, redis/3 fails. The `nil` value may be embedded inside lists or maps.
  - A number Returned if the server replies an integer (":Int"), double (",Num") or big integer ("(Num")
  - A string Returned on a bulk reply. Bulk replies are supposed to be in UTF-8 encoding. The the bulk reply starts with "`\`u0000T`\`u0000" it is supposed to be a Prolog term. Note that this intepretation means it is not possible to read arbitrary binary blobs.
  - A list of replies. A list may also contain `nil`. If Reply as a whole would be `nil` the call fails.
  - A list of pairs. This is returned for the redis version 3 protocol "%Map". Both the key and value respect the same rules as above.

  Redis bulk replies are translated depending on the `as` Type as explained above.

  string
  string(Encoding)
    Create a SWI-Prolog string object interpreting the blob as following Encoding. Encoding is a restricted set of SWI-Prolog's encodings: `bytes` (`iso_latin_1`), `utf8` and `text` (the current locale translation).
  atom
  atom(Encoding)
    As above, producing an atom.
  codes
  codes(Encoding)
    As above, producing a list of integers (Unicode code points)
  chars
  chars(Encoding)
    As above, producing a list of one-character atoms.
  integer
  float
  rational
  number
    Interpret the bytes as a string representing a number. If the string does not represent a number of the requested type a `type_error(Type, String)` is raised.
  tagged_integer
    Same as integer, but demands the value to be between the Prolog flags `min_tagged_integer` and `max_tagged_integer`, allowing the value to be used as a dict key.
  auto
    Same as `auto(atom, number)`
  auto(AsText, AsNumber)
    If the bulk string confirms the syntax of AsNumber, convert the value to the requested numberical type. Else convert the value to text according to AsText. This is similar to the Prolog predicate name/2.
  dict_key
    Alias for `auto(atom,tagged_integer)`. This allows the value to be used as a key for a SWI-Prolog dict.
  pairs(AsKey, AsValue)
    Convert a map or array of even length into pairs for which the key satisfies AsKey and the value AsValue. The `pairs` type can also be applied to a Redis array. In this case the array length must be even. This notably allows fetching a Redis hash as pairs using `HGETALL` using version 2 of the Redis protocol.
  dict(AsKey, AsValue)
    Similar to `pairs(AsKey, AsValue)`, but convert the resulting pair list into a SWI-Prolog dict. AsKey must convert to a valid dict key, i.e., an atom or tagged integer. See `dict_key`.
  dict(AsValue)
    Shorthand for `dict(dict_key, AsValue)`.

  Here are some simple examples

      ?- redis(default, set(a, 42), X).
      X = status("OK").
      ?- redis(default, get(a), X).
      X = "42".
      ?- redis(default, get(a), X as integer).
      X = 42.
      ?- redis(default, get(a), X as float).
      X = 42.0.
      ?- redis(default, set(swipl:version, 8)).
      true.
      ?- redis(default, incr(swipl:version), X).
      X = 9.

  Errors
    `redis_error(Code, String)`
redis(+Request)
  Connect to the default redis server, call redist/3 using Request, disconnect and print the result. This predicate is intended for interactive usage.
[det]redis_write(+Redis, +Command)
[det]redis_read(+Redis, -Reply)
  Write command and read replies from a Redis server. These are building blocks for subscribing to event streams.
[det]redis_get_list(+Redis, +Key, -List)
[det]redis_get_list(+Redis, +Key, +ChunkSize, -List)
  Get the content of a Redis list in List. If ChunkSize is given and smaller than the list length, List is returned as a lazy list. The actual values are requested using redis `LRANGE` requests. Note that this results in O(N`^`2) complexity. Using a lazy list is most useful for relatively short lists holding possibly large items.

  Note that values retrieved are strings, unless the value was added using `Term as prolog`.

  See also
    lazy_list/2 for a discussion on the difference between lazy lists and normal lists.
[det]redis_set_list(+Redis, +Key, +List)
  Associate a Redis key with a list. As Redis has no concept of an empty list, if List is `[]`, Key is deleted. Note that key values are always strings in Redis. The same conversion rules as for redis/1-3 apply.
[det]redis_get_hash(+Redis, +Key, -Data:dict)
[det]redis_set_hash(+Redis, +Key, +Data:dict)
  Put/get a Redis hash as a Prolog dict. Putting a dict first deletes Key. Note that in many cases applications will manage Redis hashes by key. redis_get_hash/3 is notably a user friendly alternative to the Redis `HGETALL` command. If the Redis hash is not used by other (non-Prolog) applications one may also consider using the `Term as prolog` syntax to store the Prolog dict as-is.
[det]redis_array_dict(?Array, ?Tag, ?Dict)
  Translate a Redis reply representing hash data into a SWI-Prolog dict. Array is either a list of alternating keys and values or a list of pairs. When translating to an array, this is always a list of alternating keys and values.

  Tag | is the SWI-Prolog dict tag.

[det]redis_scan(+Redis, -LazyList, +Options)
[det]redis_sscan(+Redis, +Set, -LazyList, +Options)
[det]redis_hscan(+Redis, +Hash, -LazyList, +Options)
[det]redis_zscan(+Redis, +Set, -LazyList, +Options)
  Map the Redis `SCAN`, `SSCAN`, `HSCAN` and ZSCAN‘commands into a lazy list. For redis_scan/3 and redis_sscan/4 the result is a list of strings. For redis_hscan/4 and redis_zscan/4, the result is a list of pairs. Options processed:
  match(Pattern)
    Adds the `MATCH` subcommand, only returning matches for Pattern.
  count(Count)
    Adds the `COUNT` subcommand, giving a hint to the size of the chunks fetched.
  type(Type)
    Adds the `TYPE` subcommand, only returning answers of the indicated type.
  See also
    lazy_list/2.
[nondet]redis_current_command(+Redis, ?Command)
[nondet]redis_current_command(+Redis, ?Command, -Properties)
  True when Command has Properties. Fails if Command is not defined. The redis_current_command/3 version returns the command argument specification. See Redis documentation for an explanation.
[nondet]redis_property(+Redis, ?Property)
  True if Property is a property of the Redis server. Currently uses `redis(info, String)` and parses the result. As this is for machine usage, properties names *_human are skipped.
[det]redis_subscribe(+Redis, +Channels, -Id, +Options)
  Subscribe to one or more Redis PUB/SUB channels. This predicate creates a thread using thread_create/3 with the given Options. Once running, the thread listens for messages. The message content is a string or Prolog term as described in redis/3. On receiving a message, the following message is broadcasted:

      redis(Id, Channel, Data)

  If redis_unsubscribe/2 removes the last subscription, the thread terminates.

  To simply print the incomming messages use e.g.

      ?- listen(redis(_, Channel, Data),
                format('Channel ~p got ~p~n', [Channel,Data])).
      true.
      ?- redis_subscribe(default, test, Id, []).
      Id = redis_pubsub_3,
      ?- redis(publish(test, "Hello world")).
      Channel test got "Hello world"
      1
      true.

  Id | is the thread identifier of the listening thread. Note that the Options `alias(Name)` can be used to get a system wide name.

[det]redis_subscribe(+Id, +Channels)
[det]redis_unsubscribe(+Id, +Channels)
  Add/remove channels from for the subscription. If no subscriptions remain, the listening thread terminates.

  Channels | is either a single channel or a list thereof. Each channel specification is either an atom or a term‘A:B:...`, where all parts are atoms.

redis_current_subscription(?Id, ?Channels)
  True when a PUB/SUB subscription with Id is listening on Channels.

## 3 library(redis_streams): Using Redis streams

See also
  https://redis.io/topics/streams-intro

A Redis stream is a set of messages consisting of key-value pairs that are identified by a time and sequence number. Streams are powerful objects that can roughly be used for three purposes:

- Maintain and query a log of events, i.e., a timeline.
- Provide an alternative to Redis' publish/subscribe API that ensures messages get delivered by all clients even if they are offline at the moment an event is published.
- Distribute messages over a group of clients. This mode assigns messages to clients in a round-robin fashion. Clients confirm a specific message is handled. Living clients can inspect the stream for possibly dead clients and migrate the pending messages to other clients.

This library abstracts the latter two scenarios. The main predicates are

- xadd/4 to add to a stream
- xlisten/3 to read and broadcast messages from a stream
- xlisten_group/5 to act as a consumer in a consumer group.

xstream_set(+Redis, +Key, +Option)
  Set an option on for Key on Redis. Currently supports:
  maxlen(+Count)
    Make xadd/4 add a `MAXLEN ~` Count option to the `XADD` command, capping the length of the stream. See also Redis as a message brokering system (section 1.3)
[det]xadd(+Redis, +Key, ?Id, +Data:dict)
  Add a message to a the stream Key on Redis. The length of the stream can be capped using the xstream_set/3 option `maxlen(Count)`. If Id is unbound, generating the id is left to the server and Id is unified with the returned id. The returned id is a string consisting of the time stamp in milliseconds and a sequence number. See Redis docs for details.
xlisten(+Redis, +Streams, +Options)
  Listen using `XREAD` on one or more Streams on the server Redis. For each message that arrives, call broadcast/1, where Data is a dict representing the message.

      broadcast(redis(Redis, Stream, Id, Data))

  Options:

  count(+Count)
    Process at most Count messages per stream for each request.
  start(+Start)
    Normally either `0` to start get all messages from the epoch or `$` to get messages starting with the last. Default is `$`.
  starts(+List)
    May be used as an alternative to the start/1 option to specify the start for each stream. This may be used to restart listening if the application remembers the last processed id.

  Note that this predicate does not terminate. It is normally executed in a thread. The following call listens to the streams `key1` and `key2` on the default Redis server. Using `reconnect(true)`, the client will try to re-establish a connection if the collection got lost.

      ?- redis_connect(default, C, [reconnect(true)]),
         thread_create(xlisten(C, [key1, key2], [start($)]),
                       _, [detached(true)]).

  Redis | is either a Redis server name (see redis_server/3) or an open connection. If it is a server name, a new connection is opened that is closed if xlisten/3 completes.

  See also
    redis_subscribe/2 implements the classical pub/sub system of Redis that does not have any memory.
xlisten_group(+Redis, +Group, +Consumer, +Streams, +Options)
  Listen as Consumer to Group. This is similar to xlisten/3, with the following differences:

  - Instead of using broadcast/1, broadcast_request/1 is used and the message is only considered processed if broadcast_request/1 succeeds. If the message is handled with success, an `XACK` is sent to the server.

  Options processed:

  block(+Seconds)
    Causes `XREADGROUP` to return with timeout when no messages arrive within Seconds. On a timeout, xidle_group/5 is called which will try to handle messages to other consumers pending longer than Seconds. Choosing the time depends on the application. Notably:

    - Using a time shorter than the required processing time will make the job migrate from consumer to consumer until `max_deliveries(Count)` is exceeded. Note that the original receiver does not notice that the job is claimed and thus multiple consumers may ultimately answer the message.
    - Using a too long time causes an unnecessarily long delay if a node fails.
  max_deliveries(+Count)
    Re-deliver (using `XCLAIM`) a message max Count times. Exceeding this calls xhook/2. Default Count is `3`.
  max_claim(+Count)
    Do not claim more than Count messages during a single idle action. Default is `10`.
xconsumer_stop(+Leave)
  May be called from a consumer listener to stop the consumer. This predicate throws the exception `redis(stop(Leave))`, which is caught by xlisten_group/5.
[multifile]xhook(+Stream, +Event)
  This multifile predicate is called on certain stream events. Defined events are:
  delivery_failed(Id, Group, Delivered)
    A message was delivered more than specified by max_deliveries/1 of xlisten_group/5. Id is the message id, Group the group and Delivered the current delivery count. If the hooks fails, the message is acknowledged using `XACK`. From introduction to streams:

    > "So once the deliveries counter reaches a given large number that you chose, it is probably wiser to put such messages in another stream and send a notification to the system administrator. This is basically the way that Redis streams implement the concept of the dead letter."

# Index

?
redis/1
redis/2
redis/3
redis_array_dict/3
redis_connect/1
redis_connect/3
redis_current_command/2
redis_current_command/3
redis_current_subscription/2
redis_disconnect/1
redis_disconnect/2
redis_get_hash/3
redis_get_list/3
redis_get_list/4
redis_hscan/4
redis_property/2
redis_read/2
redis_scan/3
redis_server/3
redis_set_hash/3
redis_set_list/3
redis_sscan/4
redis_subscribe/2
redis_subscribe/4
redis_unsubscribe/2
redis_write/2
redis_zscan/4
xadd/4
xconsumer_stop/1
xhook/2
xlisten/3
xlisten_group/5
xstream_set/3