Once started, the MQI listens for TCP/IP or Unix Domain Socket connections and authenticates them using the password provided (or created depending on options) before processing any messages. The messages processed by the MQI are described below.

For debugging, the server outputs traces using the debug/3 predicate so that the server operation can be observed by using the debug/1 predicate. Run the following commands to see them:

• debug(mqi(protocol)): Traces protocol messages to show the flow of commands and connections. It is designed to avoid filling the screen with large queries and results to make it easier to read.
• debug(mqi(query)): Traces messages that involve each query and its results. Therefore it can be quite verbose depending on the query. Options

  Options is a list containing any combination of the following options. When used in the Prolog top level (i.e. in Standalone Mode), these are specified as normal Prolog options like this:

  mqi_start([unix_domain_socket(Socket), password('a password')])

  When using "Embedded Mode" they are passed using the same name but as normal command line arguments like this:

  swipl mqi --write_connection_values=true
            --password="a password" --create_unix_domain_socket=true

  Note the use of quotes around values that could confuse command line processing like spaces (e.g. "a password") and that unix_domain_socket(Variable) is written as --create_unix_domain_socket=true on the command line. See below for more information.

• port(?Port) The TCP/IP port to bind to on localhost. This option is ignored if the unix_domain_socket/1 option is set. Port is either a legal TCP/IP port number (integer) or a variable term like Port. If it is a variable, it causes the system to select a free port and unify the variable with the selected port as in tcp_bind/2. If the option write_connection_values(true) is set, the selected port is output to STDOUT followed by \n on startup to allow the client language library to retrieve it in "Embedded Mode".
• unix_domain_socket(?Unix_Domain_Socket_Path_And_File) If set, Unix Domain Sockets will be used as the way to communicate with the server. Unix_Domain_Socket_Path_And_File specifies the fully qualified path and filename to use for the socket.

  To have one generated instead (recommended), pass Unix_Domain_Socket_Path_And_File as a variable when calling from the Prolog top level and the variable will be unified with a created filename. If launching in "Embedded Mode", instead pass --create_unix_domain_socket=true since there isn't a way to specify variables from the command line. When generating the file, a temporary directory will be created using tmp_file/2 and a socket file will be created within that directory following the below requirements. If the directory and file are unable to be created for some reason, mqi_start/1 fails.

  Regardless of whether the file is specified or generated, if the option write_connection_values(true) is set, the fully qualified path to the generated file is output to STDOUT followed by \n on startup to allow the client language library to retrieve it.

  Specifying a file to use should follow the same guidelines as the generated file:

  • If the file exists when the MQI is launched, it will be deleted.
  • The Prolog process will attempt to create and, if Prolog exits cleanly, delete this file (and directory if it was created) when the MQI closes. This means the directory from a specified file must have the appropriate permissions to allow the Prolog process to do so.
  • For security reasons, the filename should not be predictable and the directory it is contained in should have permissions set so that files created are only accessible to the current user.
  • The path must be below 92 bytes long (including null terminator) to be portable according to the Linux documentation.
• password(?Password) The password required for a connection. If not specified (recommended), the MQI will generate one as a Prolog string type since Prolog atoms are globally visible (be sure not to convert to an atom for this reason). If Password is a variable it will be unified with the created password. Regardless of whether the password is specified or generated, if the option write_connection_values(true) is set, the password is output to STDOUT followed by \n on startup to allow the client language library to retrieve it. This is the recommended way to integrate the MQI with a language as it avoids including the password as source code. This option is only included so that a known password can be supplied for when the MQI is running in Standalone Mode.
• query_timeout(+Seconds) Sets the default time in seconds that a query is allowed to run before it is cancelled. This can be overridden on a query by query basis. If not set, the default is no timeout (-1).
• pending_connections(+Count) Sets the number of pending connections allowed for the MQI as in tcp_listen/2. If not provided, the default is 5.
• run_server_on_thread(+Run_Server_On_Thread) Determines whether mqi_start/1 runs in the background on its own thread or blocks until the MQI shuts down. Must be missing or set to true when running in "Embedded Mode" so that the SWI Prolog process can exit properly. If not set, the default is true.
• server_thread(?Server_Thread) Specifies or retrieves the name of the thread the MQI will run on if run_server_on_thread(true). Passing in an atom for Server_Thread will only set the server thread name if run_server_on_thread(true). If Server_Thread is a variable, it is unified with a generated name.
• write_connection_values(+Write_Connection_Values) Determines whether the server writes the port (or generated Unix Domain Socket) and password to STDOUT as it initializes. Used by language libraries to retrieve this information for connecting. If not set, the default is false.
• write_output_to_file(+File) Redirects STDOUT and STDERR to the file path specified. Useful for debugging the MQI when it is being used in "Embedded Mode". If using multiple MQI instances in one SWI Prolog instance, only set this on the first one. Each time it is set the output will be redirected.