-----------------
Monitoring tables
-----------------

  Function:
    Allow to monitor server-side activity happening inside a particular database.

  Concept:    
    The engine offers a set of so called "virtual" tables that provides the user
    with a snapshot of the current activity within the given database. The word
    "virtual" means that the table data doesn't exist until explicitly asked for.
    However, its metadata is stable and can be retrieved from the schema. Virtual
    monitoring tables exist only in ODS 11.1 (and higher) databases, so a
    migration via backup/restore is required in order to use this feature.

    The key term of the monitoring feature is an activity snapshot. It represents
    the current state of the database, consisting of various information about
    the database itself, active attachments and users, transactions, prepared and
    running statements, etc. A snapshot is created the first time any of the
    monitoring tables is being selected from in the given transaction and it's
    preserved until the transaction ends, so multiple queries (e.g. master-detail
    ones) will always return the consistent view of the data. In other words, the
    monitoring tables always behave like a snapshot (aka consistency) transaction,
    even if the host transaction has been started with another isolation level.
    To refresh the snapshot, the current transaction should be finished and the
    monitoring tables should be queried in the new transaction context. Creation
    of a snapshot is usually quite fast operation, but some delay should be
    expected under high load (especially in the Classic Server). 

    A valid database connection is required in order to retrieve the monitoring
    data. The monitoring tables return information about the attached database
    only. If multiple databases are being accessed on the server, each of them
    has to be connected to and monitored separately.

    System variables CURRENT_CONNECTION and CURRENT_TRANSACTION could be used
    to select data about the current (for the caller) connection and transaction
    respectively. These variables correspond to the ID columns of the appropriate
    monitoring tables.

  Security:
    Complete database monitoring is available to SYSDBA and a database owner.
    Regular users are restricted to the information about their own attachments
    only (other attachments are invisible for them).

  Author:
    Dmitry Yemanov <dimitr at firebirdsql dot org>

  Scope:
    DSQL and PSQL

  ODS 11.1 metadata:
    MON$DATABASE (connected database)
      - MON$DATABASE_NAME (database pathname or alias)
      - MON$PAGE_SIZE (page size)
      - MON$ODS_MAJOR (major ODS version)
      - MON$ODS_MINOR (minor ODS version)
      - MON$OLDEST_TRANSACTION (OIT number)
      - MON$OLDEST_ACTIVE (OAT number)
      - MON$OLDEST_SNAPSHOT (OST number)
      - MON$NEXT_TRANSACTION (next transaction number)
      - MON$PAGE_BUFFERS (number of pages allocated in the cache)
      - MON$SQL_DIALECT (SQL dialect of the database)
      - MON$SHUTDOWN_MODE (current shutdown mode)
          0: online
          1: multi-user shutdown
          2: single-user shutdown
          3: full shutdown
      - MON$SWEEP_INTERVAL (sweep interval)
      - MON$READ_ONLY (read-only flag)
      - MON$FORCED_WRITES (sync writes flag)
      - MON$RESERVE_SPACE (reserve space flag)
      - MON$CREATION_DATE (creation date/time)
      - MON$PAGES (number of pages allocated on disk)
      - MON$BACKUP_STATE (current physical backup state)
          0: normal
          1: stalled
          2: merge
      - MON$STAT_ID (statistics ID)

    MON$ATTACHMENTS (connected attachments)
      - MON$ATTACHMENT_ID (attachment ID)
      - MON$SERVER_PID (server process ID)
      - MON$STATE (attachment state)
          0: idle
          1: active
      - MON$ATTACHMENT_NAME (connection string)
      - MON$USER (user name)
      - MON$ROLE (role name)
      - MON$REMOTE_PROTOCOL (remote protocol name)
      - MON$REMOTE_ADDRESS (remote address)
      - MON$REMOTE_PID (remote client process ID)
      - MON$REMOTE_PROCESS (remote client process pathname)
      - MON$CHARACTER_SET_ID (attachment character set)
      - MON$TIMESTAMP (connection date/time)
      - MON$GARBAGE_COLLECTION (garbage collection flag)
      - MON$STAT_ID (statistics ID)

    MON$TRANSACTIONS (started transactions)
      - MON$TRANSACTION_ID (transaction ID)
      - MON$ATTACHMENT_ID (attachment ID)
      - MON$STATE (transaction state)
          0: idle
          1: active
      - MON$TIMESTAMP (transaction start date/time)
      - MON$TOP_TRANSACTION (top transaction)
      - MON$OLDEST_TRANSACTION (local OIT number)
      - MON$OLDEST_ACTIVE (local OAT number)
      - MON$ISOLATION_MODE (isolation mode)
          0: consistency
          1: concurrency
          2: read committed record version
          3: read committed no record version
      - MON$LOCK_TIMEOUT (lock timeout)
          -1: infinite wait
          0: no wait
          N: timeout N
      - MON$READ_ONLY (read-only flag)
      - MON$AUTO_COMMIT (auto-commit flag)
      - MON$AUTO_UNDO (auto-undo flag)
      - MON$STAT_ID (statistics ID)

    MON$STATEMENTS (prepared statements)
      - MON$STATEMENT_ID (statement ID)
      - MON$ATTACHMENT_ID (attachment ID)
      - MON$TRANSACTION_ID (transaction ID)
      - MON$STATE (statement state)
          0: idle
          1: active
      - MON$TIMESTAMP (statement start date/time)
      - MON$SQL_TEXT (statement text, if appropriate)
      - MON$STAT_ID (statistics ID)

    MON$CALL_STACK (call stack of active PSQL requests)
      - MON$CALL_ID (call ID)
      - MON$STATEMENT_ID (top-level DSQL statement ID)
      - MON$CALLER_ID (caller request ID)
      - MON$OBJECT_NAME (PSQL object name)
      - MON$OBJECT_TYPE (PSQL object type)
      - MON$TIMESTAMP (request start date/time)
      - MON$SOURCE_LINE (SQL source line number)
      - MON$SOURCE_COLUMN (SQL source column number)
      - MON$STAT_ID (statistics ID)

    MON$IO_STATS (I/O statistics)
      - MON$STAT_ID (statistics ID)
      - MON$STAT_GROUP (statistics group)
          0: database
          1: attachment
          2: transaction
          3: statement
          4: call
      - MON$PAGE_READS (number of page reads)
      - MON$PAGE_WRITES (number of page writes)
      - MON$PAGE_FETCHES (number of page fetches)
      - MON$PAGE_MARKS (number of page marks)

    MON$RECORD_STATS (record-level statistics)
      - MON$STAT_ID (statistics ID)
      - MON$STAT_GROUP (statistics group)
          0: database
          1: attachment
          2: transaction
          3: statement
          4: call
      - MON$RECORD_SEQ_READS (number of records read sequentially)
      - MON$RECORD_IDX_READS (number of records read via an index)
      - MON$RECORD_INSERTS (number of inserted records)
      - MON$RECORD_UPDATES (number of updated records)
      - MON$RECORD_DELETES (number of deleted records)
      - MON$RECORD_BACKOUTS (number of backed out records)
      - MON$RECORD_PURGES (number of purged records)
      - MON$RECORD_EXPUNGES (number of expunged records)

  ODS 11.2 metadata:
    MON$MEMORY_USAGE (current memory usage)
      - MON$STAT_ID (statistics ID)
      - MON$STAT_GROUP (statistics group)
          0: database
          1: attachment
          2: transaction
          3: statement
          4: call
      - MON$MEMORY_USED (number of bytes currently in use)
      - MON$MEMORY_ALLOCATED (number of bytes currently allocated at the OS level)
      - MON$MAX_MEMORY_USED (maximum number of bytes used by this object)
      - MON$MAX_MEMORY_ALLOCATED (maximum number of bytes allocated from OS by this object)

    MON$CONTEXT_VARIABLES (known context variables)
      - MON$ATTACHMENT_ID (attachment ID)
      - MON$TRANSACTION_ID (transaction ID)
      - MON$VARIABLE_NAME (name of context variable)
      - MON$VARIABLE_VALUE (value of context variable)

  Notes:
    1) Textual descriptions of all "state" and "mode" values can be found
       in the system table RDB$TYPES

    2) For table MON$ATTACHMENTS:
      - columns MON$REMOTE_PID and MON$REMOTE_PROCESS contains non-NULL values
        only if the client library has version 2.1 or higher
      - column MON$REMOTE_PROCESS can contain a non-pathname value
        if an application has specified a custom process name via DPB

    3) For table MON$STATEMENTS:
      - column MON$SQL_TEXT contains NULL for GDML statements
      - columns MON$TRANSACTION_ID and MON$TIMESTAMP contain valid values
        for active statements only

    4) For table MON$CALL_STACK:
      - column MON$STATEMENT_ID groups call stacks by the top-level DSQL statement
        that initiated the call chain. This ID represents an active statement
        record in the table MON$STATEMENTS.
      - columns MON$SOURCE_LINE and MON$SOURCE_COLUMN contain line/column information
        related to the PSQL statement being currently executed

    5) For table MON$MEMORY_USAGE:
      - the "used" values represent high-level memory allocations, i.e. the ones
        performed by the engine from its pools. They are useful to investigate unexpected
        memory consumptions and find the "guilty" objects (attachments, procedures, etc),
        as well as trace memory leaks.
      - the "allocated" values represent low-level memory allocations, i.e. the ones
        performed by the Firebird memory manager. This means bytes really allocated from OS,
        thus allowing to monitor the physical memory consumption. Please note that not every
        record has these columns populated with non-zero values. Small allocations don't go
        to the OS level, they're redirected to the database memory pool instead. So usually
        only MON$DATABASE and memory-bound objects point to non-zero "allocated" values.
      - the counter set linked to a record in MON$DATABASE reports the memory shared among
        all attachments. In Classic and SuperClassic, these counters are zero meaning no
        shared cache in these architectures.

    6) For table MON$CONTEXT_VARIABLES:
      - column MON$ATTACHMENT_ID contains a valid ID only for session-level context variables.
        Transaction-level ones have this field set to NULL.
      - column MON$TRANSACTION_ID contains a valid ID only for transaction-level context variables.
        Session-level ones have this field set to NULL.

  Example(s):
    1) Retrieve IDs of all CS processes loading CPU at the moment:
        SELECT MON$SERVER_PID
        FROM MON$ATTACHMENTS
        WHERE MON$ATTACHMENT_ID <> CURRENT_CONNECTION
          AND MON$STATE = 1

    2) Retrieve information about client applications:
        SELECT MON$USER, MON$REMOTE_ADDRESS, MON$REMOTE_PID, MON$TIMESTAMP
        FROM MON$ATTACHMENTS
        WHERE MON$ATTACHMENT_ID <> CURRENT_CONNECTION

    3) Get isolation level of the current transaction:
        SELECT MON$ISOLATION_MODE
        FROM MON$TRANSACTIONS
        WHERE MON$TRANSACTION_ID = CURRENT_TRANSACTION

    4) Get statements that are currently active:
        SELECT ATT.MON$USER, ATT.MON$REMOTE_ADDRESS, STMT.MON$SQL_TEXT, STMT.MON$TIMESTAMP
        FROM MON$ATTACHMENTS ATT
          JOIN MON$STATEMENTS STMT ON ATT.MON$ATTACHMENT_ID = STMT.MON$ATTACHMENT_ID
        WHERE ATT.MON$ATTACHMENT_ID <> CURRENT_CONNECTION
          AND STMT.MON$STATE = 1

    5) Retrieve call stacks for all connections:
        WITH RECURSIVE
          HEAD AS
        (
          SELECT CALL.MON$STATEMENT_ID, CALL.MON$CALL_ID, CALL.MON$OBJECT_NAME, CALL.MON$OBJECT_TYPE
          FROM MON$CALL_STACK CALL
          WHERE CALL.MON$CALLER_ID IS NULL
          UNION ALL
          SELECT CALL.MON$STATEMENT_ID, CALL.MON$CALL_ID, CALL.MON$OBJECT_NAME, CALL.MON$OBJECT_TYPE
          FROM MON$CALL_STACK CALL
            JOIN HEAD ON CALL.MON$CALLER_ID = HEAD.MON$CALL_ID
        )
        SELECT MON$ATTACHMENT_ID, MON$OBJECT_NAME, MON$OBJECT_TYPE
        FROM HEAD
          JOIN MON$STATEMENTS STMT ON STMT.MON$STATEMENT_ID = HEAD.MON$STATEMENT_ID
        WHERE STMT.MON$ATTACHMENT_ID <> CURRENT_CONNECTION

    6) Enumerate all session-level context variables for the current connection:
        SELECT VAR.MON$VARIABLE_NAME, VAR.MON$VARIABLE_VALUE
        FROM MON$CONTEXT_VARIABLES VAR
        WHERE VAR.MON$ATTACHMENT_ID = CURRENT_CONNECTION

    7) Report top 10 statements ranked by their memory usage:
        SELECT FIRST 10 STMT.MON$ATTACHMENT_ID, STMT.MON$SQL_TEXT, MEM.MON$MEMORY_USED
        FROM MON$MEMORY_USAGE MEM
          NATURAL JOIN MON$STATEMENTS STMT
        ORDER BY MEM.MON$MEMORY_USED DESC

--------------------------------------
Modifications of the monitoring tables
--------------------------------------

    Monitoring tables also allow some special administration activities, in particular:
    cancelling running statements and terminating client sessions. This is done via deletes
    from tables MON$STATEMENTS and MON$ATTACHMENTS respectively. Deletes from other tables,
    as well as inserts/updates issued against them, are prohibited.

  Notes:
    1) If there are no statements currently running by the client, then the cancellation
       attempt becomes a void operation. Once cancelled, the execute/fetch API call returns
       the isc_cancelled error code. Any subsequent operations are allowed.

    2) If there are active transactions in the connection being terminated, their activity
       is immediately cancelled and they're rolled back. Once terminated, the client session
       receives the isc_att_shutdown error code. Subsequent attempts to use this connection
       handle will cause network read/write errors.

  Example(s):
    1) Cancel current activity of connection #32:
        DELETE FROM MON$STATEMENTS WHERE MON$ATTACHMENT_ID = 32

    2) Disconnect everybody but ourselves:
        DELETE FROM MON$ATTACHMENTS WHERE MON$ATTACHMENT_ID <> CURRENT_CONNECTION


--------------
Under the hood
--------------

    The monitoring implementation is built around two corner stones: shared memory and
    notifications.

    All server processes share some region of memory where the current activity information
    is stored. This information consists of multiple variable-length items describing the
    various activity details. All items that belong to the same process are grouped into a
    single cluster, so that they can be processed as a whole.

    The monitoring information is not populated/collected in real time. Instead, server
    processes write their data into the shared memory only when explicitly asked to. When doing
    so, the old clusters are being replaced with newer ones. When the shared memory region is
    being read, the reading process scans all the clusters and performs the garbage collection:
    clusters that belong to dead processes are removed and the shared memory space is compacted.

    Every server process has a flag that indicates its ability to react to someone's monitoring
    request as soon as it arrives. When some user connection runs a query against some
    monitoring table, the worker process of that connection sends a broadcast notification to
    other processes requesting an up-to-date information. Those processes react to this request
    by updating their clusters inside the shared memory region and clearing their "ready" flags.
    Once the every notified process has finished, the requesting one reads the shared memory
    region, filters the necessary tags based on its user permissions, transforms the internal
    representation into records and fields and populates the in-memory monitoring tables cache.

    Processes that were idle since the last monitoring exchange have their "ready" flag clear,
    thus indicating that they have nothing to update in the shared memory. This way they're
    excluded from the next roundtrip. As soon as something significant changed inside the
    process, the flag is set and this process starts responding to the monitoring requests
    again.

    The requester holds an exclusive lock while coordinating the write/read operations. This lock
    affects the currently active user connections as well as the connections being established.
    Multiple simultaneous monitoring requests are serialized.


----------------------------
Limitations and known issues
----------------------------

    1) In a heavily loaded system running Classic, monitoring requests may take noticeable time
       to execute. In the meantime, other activity (both running statements and new connection
       attempts) may be blocked until the monitoring request completes.

       Improved since FB v2.1.2.

    2) Monitoring requests may sometimes fail due to the out-of-memory condition, or cause other
       worker processes to swap. This is caused by the fact that the every record in MON$STATEMENTS
       has a blob MON$SQL_TEXT which is created for the duration of the monitoring transaction.
       Prior to FB v2.5, every blob occupied <page size> bytes of memory even if its contents is
       in fact smaller. So, with a huge number of prepared statements in the system, it becomes
       possible to get this failure.
       
       Another possible reason could be the temporary (very short in practice) growth of the
       transaction pool which caches the monitoring data while merging the clusters into a single
       fragment.

       Improved since FB v2.5.0.
