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\section{library(redis): Redis client}

\label{sec:redis}

This library is a client to \href{https://redis.io}{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 \predref{redis_server}{3}. Subsequenly, \predref{redis}{2}-3 is used to issue
commands on the server. For example:

\begin{code}
?- redis_server(default, redis:6379, [password("secret")]).
?- redis(default, set(user, "Bob")).
?- redis(default, get(user), User).
User = "Bob"
\end{code}

\vspace{0.7cm}

\begin{description}
    \predicate[det]{redis_server}{3}{+ServerName, +Address, +Options}
Register a redis server without connecting to it. The \arg{ServerName}
acts as a lazy connection alias. Initially the \arg{ServerName} \const{default}
points at \verb$localhost:6379$ with no connect options. The \const{default}
server is used for \predref{redis}{1} and \predref{redis}{2} and may be changed using this
predicate. \arg{Options} are described with \predref{redis_connect}{3}.

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

    \predicate[det]{redis_connect}{1}{-Connection}
\nodescription
    \predicate[det]{redis_connect}{3}{+Address, -Connection, +Options}
\nodescription
    \predicate[det]{redis_connect}{3}{-Connection, +Host, +Port}
Connect to a redis server. The main mode is \verb$redis_connect(+Address, -Connection, +Options)$. \predref{redis_connect}{1} is equivalent to
\verb$redis_connect(localhost:6379, Connection, [])$. \arg{Options}:

\begin{description}
    \termitem{reconnect}{+Boolean}
If \const{true}, try to reconnect to the service when the connection
seems lost. Default is \const{true} for connections specified using
\predref{redis_server}{3} and \const{false} for explictly opened connections.
    \termitem{user}{+User}
If \verb$version(3)$ and \verb$password(Password)$ are specified, these
are used to authenticate using the \arg{HELLO} command.
    \termitem{password}{+Password}
Authenticate using \arg{Password}
    \termitem{version}{+Version}
Specify the connection protocol version. Initially this is
version 2. Redis 6 also supports version 3. When specified
as \verb$3$, the \arg{HELLO} command is used to upgrade the protocol.
    \termitem{tls}{true}
When specified, initiate a TLS connection. If this option is
specified we must also specify the \const{cacert}, \const{key} and \const{cert}
options.
    \termitem{cacert}{+File}
CA Certificate file to verify with.
    \termitem{cert}{+File}
Client certificate to authenticate with.
    \termitem{key}{+File}
Private key file to authenticate with.
    \termitem{sentinels}{+ListOfAddresses}
Used together with an \arg{Address} of the form \verb$sentinel(MasterName)$
to enable contacting a network of Redis servers guarded by a
sentinel network.
    \termitem{sentinel_user}{+User}
    \termitem{sentinel_password}{+Password}
Authentication information for the senitels. When omitted we
try to connect withour authentication.
\end{description}

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

\begin{arguments}
\arg{Address} & is a term \arg{Host}:\arg{Port}, \verb$unix(File)$ or the name of a server
registered using \predref{redis_server}{3}. The latter realises a \textit{new}
connection that is typically used for blocking redis commands such
as listening for published messages, waiting on a list or stream. \\
\end{arguments}

\begin{tags}
    \tag{Compatibility}
\verb$redis_connect(-Connection, +Host, +Port)$ provides
compatibility to the original GNU-Prolog interface and is equivalent
to \verb$redis_connect(Host:Port, Connection, [])$.
\end{tags}

    \predicate[semidet]{tls_verify}{5}{+SSL, +ProblemCert, +AllCerts, +FirstCert, +Status}
Accept or reject the certificate verification. Similar to the
Redis command line client (\verb$redis-cli$), we accept the
certificate as long as it is signed, not verifying the hostname.

    \predicate[det]{redis_disconnect}{1}{+Connection}
\nodescription
    \predicate[det]{redis_disconnect}{2}{+Connection, +Options}
Disconnect from a redis server. The second form takes one option,
similar to \predref{close}{2}:

\begin{description}
    \termitem{force}{Force}
When \const{true} (default \const{false}), do not raise any errors if
\arg{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.
\end{description}

    \predicate[semidet]{redis}{2}{+Connection, +Request}
This predicate is overloaded to handle two types of requests. First,
it is a shorthand for \verb$redis(Connection, Command, _)$ and second, it
can be used to exploit Redis \textit{pipelines} and \textit{transactions}. The
second form is acticated if \arg{Request} is a \textit{list}. In that case, each
element of the list is either a term \verb$Command -> Reply$ or a simple
\arg{Command}. Semantically this represents a sequence of \predref{redis}{3} and
\predref{redis}{2} calls. It differs in the following aspects:

\begin{itemize}
    \item All commands are sent in one batch, after which all replies are
read. This reduces the number of \textit{round trips} and typically
greatly improves performance.
    \item If the first command is \const{multi} and the last \const{exec}, the
commands are executed as a Redis \textit{transaction}, i.e., they
are executed \textit{atomically}.
    \item If one of the commands returns an error, the subsequent commands
\textbf{are still executed}.
    \item 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.
\end{itemize}

Procedurally, the process takes the following steps:

\begin{enumerate}
    \item Send all commands
    \item Read all replies and push messages
    \item Handle all callbacks from push messages
    \item Check whether one of the replies is an error. If so,
raise this error (subsequent errors are lost)
    \item Bind all replies for the \verb$Command -> Reply$ terms.
\end{enumerate}

Examples

\begin{code}
?- redis(default,
         [ lpush(li,1),
           lpush(li,2),
           lrange(li,0,-1) -> List
         ]).
List = ["2", "1"].
\end{code}

    \predicate[semidet]{redis}{3}{+Connection, +Command, -Reply}
Execute a redis \arg{Command} on Connnection. Next, bind \arg{Reply} to the
returned result. \arg{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.

\begin{itemize}
    \item Atomic values are emitted verbatim
    \item A term A:B:... where all arguments are either atoms,
strings or integers (\textbf{no floats}) is translated into
a string \verb$"A:B:..."$. This is a common shorthand for
representing Redis keys.
    \item A term Term as prolog is emitted as "\Sneg{}u0000T\Sneg{}u0000" followed
by Term in canonical form.
    \item Any other term is emitted as \predref{write}{1}.
\end{itemize}

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

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

Redis \textit{bulk} replies are translated depending on the \const{as} \arg{Type} as
explained above.

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

Here are some simple examples

\begin{code}
?- 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.
\end{code}

\begin{tags}
    \tag{Errors}
\verb$redis_error(Code, String)$
\end{tags}

    \predicate{redis}{1}{+Request}
Connect to the default redis server, call \predref{redist}{3} using \arg{Request},
disconnect and print the result. This predicate is intended for
interactive usage.

    \predicate[det]{redis_write}{2}{+Redis, +Command}
\nodescription
    \predicate[det]{redis_read}{2}{+Redis, -Reply}
Write command and read replies from a \arg{Redis} server. These are
building blocks for subscribing to event streams.

    \predicate[det]{redis_get_list}{3}{+Redis, +Key, -List}
\nodescription
    \predicate[det]{redis_get_list}{4}{+Redis, +Key, +ChunkSize, -List}
Get the content of a \arg{Redis} list in \arg{List}. If \arg{ChunkSize} is given and
smaller than the list length, \arg{List} is returned as a \textit{lazy list}. The
actual values are requested using redis \verb$LRANGE$ requests. Note
that this results in O(N\Shat{}2) complexity. Using a lazy list is most
useful for relatively short lists holding possibly large items.

Note that values retrieved are \textit{strings}, unless the value was added
using \verb$Term as prolog$.

\begin{tags}
    \tag{See also}
\predref{lazy_list}{2} for a discussion on the difference between lazy
lists and normal lists.
\end{tags}

    \predicate[det]{redis_set_list}{3}{+Redis, +Key, +List}
Associate a \arg{Redis} key with a list. As \arg{Redis} has no concept of an
empty list, if \arg{List} is \verb$[]$, \arg{Key} is \textit{deleted}. Note that key values
are always strings in \arg{Redis}. The same conversion rules as for
\predref{redis}{1}-3 apply.

    \predicate[det]{redis_get_hash}{3}{+Redis, +Key, -Data:dict}
\nodescription
    \predicate[det]{redis_set_hash}{3}{+Redis, +Key, +Data:dict}
Put/get a \arg{Redis} hash as a Prolog dict. Putting a dict first deletes
\arg{Key}. Note that in many cases applications will manage \arg{Redis} hashes
by key. \predref{redis_get_hash}{3} is notably a user friendly alternative to
the \arg{Redis} \verb$HGETALL$ command. If the \arg{Redis} hash is not used by
other (non-Prolog) applications one may also consider using the
\verb$Term as prolog$ syntax to store the Prolog dict as-is.

    \predicate[det]{redis_array_dict}{3}{?Array, ?Tag, ?Dict}
Translate a Redis reply representing hash data into a SWI-Prolog
dict. \arg{Array} is either a list of alternating keys and values or a
list of \textit{pairs}. When translating to an array, this is always a list
of alternating keys and values.

\begin{arguments}
\arg{Tag} & is the SWI-Prolog dict tag. \\
\end{arguments}

    \predicate[det]{redis_scan}{3}{+Redis, -LazyList, +Options}
\nodescription
    \predicate[det]{redis_sscan}{4}{+Redis, +Set, -LazyList, +Options}
\nodescription
    \predicate[det]{redis_hscan}{4}{+Redis, +Hash, -LazyList, +Options}
\nodescription
    \predicate[det]{redis_zscan}{4}{+Redis, +Set, -LazyList, +Options}
Map the \arg{Redis} \verb$SCAN$, \verb$SSCAN$, \verb$HSCAN$ and \arg{ZSCAN}` commands
into a \textit{lazy list}. For \predref{redis_scan}{3} and \predref{redis_sscan}{4} the result is
a list of strings. For \predref{redis_hscan}{4} and \predref{redis_zscan}{4}, the result
is a list of \textit{pairs}. \arg{Options} processed:

\begin{description}
    \termitem{match}{Pattern}
Adds the \verb$MATCH$ subcommand, only returning matches for
\arg{Pattern}.
    \termitem{count}{Count}
Adds the \verb$COUNT$ subcommand, giving a hint to the size of the
chunks fetched.
    \termitem{type}{Type}
Adds the \verb$TYPE$ subcommand, only returning answers of the
indicated type.
\end{description}

\begin{tags}
    \tag{See also}
\predref{lazy_list}{2}.
\end{tags}

    \predicate[nondet]{redis_current_command}{2}{+Redis, ?Command}
\nodescription
    \predicate[nondet]{redis_current_command}{3}{+Redis, ?Command, -Properties}
True when \arg{Command} has \arg{Properties}. Fails if \arg{Command} is not defined.
The \predref{redis_current_command}{3} version returns the command argument
specification. See \arg{Redis} documentation for an explanation.

    \predicate[nondet]{redis_property}{2}{+Redis, ?Property}
True if \arg{Property} is a property of the \arg{Redis} server. Currently uses
\verb$redis(info, String)$ and parses the result. As this is for machine
usage, properties names *_human are skipped.

    \predicate[det]{redis_subscribe}{4}{+Redis, +Channels, -Id, +Options}
Subscribe to one or more \arg{Redis} PUB/SUB channels. This predicate
creates a thread using \predref{thread_create}{3} with the given \arg{Options}. Once
running, the thread listens for messages. The message content is a
string or Prolog term as described in \predref{redis}{3}. On receiving a
message, the following message is broadcasted:

\begin{code}
redis(Id, Channel, Data)
\end{code}

If \predref{redis_unsubscribe}{2} removes the last subscription, the thread
terminates.

To simply print the incomming messages use e.g.

\begin{code}
?- 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.
\end{code}

\begin{arguments}
\arg{Id} & is the thread identifier of the listening thread. Note that
the \arg{Options} \verb$alias(Name)$ can be used to get a system wide name. \\
\end{arguments}

    \predicate[det]{redis_subscribe}{2}{+Id, +Channels}
\nodescription
    \predicate[det]{redis_unsubscribe}{2}{+Id, +Channels}
Add/remove channels from for the subscription. If no subscriptions
remain, the listening thread terminates.

\begin{arguments}
\arg{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. \\
\end{arguments}

    \predicate{redis_current_subscription}{2}{?Id, ?Channels}
True when a PUB/SUB subscription with \arg{Id} is listening on \arg{Channels}.
\end{description}