rrives 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.

1.6 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.

tls(true)

When specified, initiate a TLS connection. If this option is specified we must also specify the cacert, key and cert options.

cacert(+File)

CA Certificate file to verify with.

cert(+File)

Client certificate to authenticate with.

key(+File)

Private key file to authenticate with.

sentinels(+ListOfAddresses)

Used together with an Address of the form sentinel(MasterName) to enable contacting a network of Redis servers guarded by a sentinel network.

sentinel_user(+User)

sentinel_password(+Password)

Authentication information for the senitels. When omitted we try to connect withour authentication.

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, []).

[semidet]tls_verify(+SSL, +ProblemCert, +AllCerts, +FirstCert, +Status)

Accept or reject the certificate verification. Similar to the Redis command line client (redis-cli), we accept the certificate as long as it is signed, not verifying the hostname.

[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 lis