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doc/README.monitoring_tables
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doc/README.monitoring_tables
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-----------------
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Monitoring tables
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-----------------
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Function:
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Allow to monitor server-side activity happening inside a particular database.
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Concept:
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The engine offers a set of so called "virtual" tables that provides the user
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with a snapshot of the current activity within the given database. The word
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"virtual" means that the table data doesn't exist until explicitly asked for.
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However, its metadata is stable and can be retrieved from the schema. Virtual
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monitoring tables exist only in ODS 11.1 (and higher) databases, so a
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migration via backup/restore is required in order to use this feature.
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The key term of the monitoring feature is an activity snapshot. It represents
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the current state of the database, consisting of various information about
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the database itself, active attachments and users, transactions, prepared and
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running statements, etc. A snapshot is created the first time any of the
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monitoring tables is being selected from in the given transaction and it's
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preserved until the transaction ends, so multiple queries (e.g. master-detail
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ones) will always return the consistent view of the data. In other words, the
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monitoring tables always behave like a snapshot (aka consistency) transaction,
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even if the host transaction has been started with another isolation level.
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To refresh the snapshot, the current transaction should be finished and the
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monitoring tables should be queried in the new transaction context. Creation
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of a snapshot is usually quite fast operation, but some delay should be
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expected under high load (especially in the Classic Server).
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A valid database connection is required in order to retrieve the monitoring
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data. The monitoring tables return information about the attached database
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only. If multiple databases are being accessed on the server, each of them
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has to be connected to and monitored separately.
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System variables CURRENT_CONNECTION and CURRENT_TRANSACTION could be used
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to select data about the current (for the caller) connection and transaction
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respectfully. These variables correspond to the ID columns of the appropriate
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monitoring tables.
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Security:
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Complete database monitoring is available to SYSDBA and a database owner.
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Regular users are restricted to the information about their own attachments
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only (other attachments are invisible for them).
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Author:
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Dmitry Yemanov <dimitr at firebirdsql dot org>
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Scope:
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DSQL and PSQL
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Metadata:
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MON$DATABASE (connected database)
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- MON$DATABASE_NAME (database pathname or alias)
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- MON$PAGE_SIZE (page size)
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- MON$ODS_MAJOR (major ODS version)
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- MON$ODS_MINOR (minor ODS version)
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- MON$OLDEST_TRANSACTION (OIT number)
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- MON$OLDEST_ACTIVE (OAT number)
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- MON$OLDEST_SNAPSHOT (OST number)
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- MON$NEXT_TRANSACTION (next transaction number)
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- MON$PAGE_BUFFERS (number of pages allocated in the cache)
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- MON$SQL_DIALECT (SQL dialect of the database)
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- MON$SHUTDOWN_MODE (current shutdown mode)
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0: online
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1: multi-user shutdown
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2: single-user shutdown
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3: full shutdown
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- MON$SWEEP_INTERVAL (sweep interval)
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- MON$READ_ONLY (read-only flag)
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- MON$FORCED_WRITES (sync writes flag)
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- MON$RESERVE_SPACE (reserve space flag)
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- MON$CREATION_DATE (creation date/time)
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- MON$PAGES (number of pages allocated on disk)
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MON$ATTACHMENTS (connected attachments)
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- MON$ATTACHMENT_ID (attachment ID)
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- MON$SERVER_PID (server process ID)
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- MON$ATTACHMENT_STATE (attachment state)
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0: idle
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1: active
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- MON$ATTACHMENT_NAME (connection string)
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- MON$USER (user name)
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- MON$ROLE (role name)
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- MON$REMOTE_PROTOCOL (remote protocol name)
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- MON$REMOTE_ADDRESS (remote address)
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- MON$REMOTE_PID (remote client process ID)
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- MON$CHARACTER_SET_ID (attachment character set)
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- MON$TIMESTAMP (connection date/time)
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- MON$GARBAGE_COLLECTION (garbage collection flag)
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MON$TRANSACTIONS (started transactions)
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- MON$TRANSACTION_ID (transaction ID)
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- MON$ATTACHMENT_ID (attachment ID)
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- MON$TRANSACTION_STATE (transaction state)
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0: idle
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1: active
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- MON$TIMESTAMP (transaction start date/time)
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- MON$TOP_TRANSACTION (top transaction)
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- MON$OLDEST_TRANSACTION (local OIT number)
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- MON$OLDEST_ACTIVE (local OAT number)
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- MON$ISOLATION_MODE (isolation mode)
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0: consistency
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1: concurrency
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2: read committed record version
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3: read committed no record version
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- MON$LOCK_TIMEOUT (lock timeout)
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-1: infinite wait
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0: no wait
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N: timeout N
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- MON$READ_ONLY (read-only flag)
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- MON$AUTO_COMMIT (auto-commit flag)
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- MON$AUTO_UNDO (auto-undo flag)
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MON$STATEMENTS (prepared statements)
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- MON$STATEMENT_ID (statement ID)
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- MON$ATTACHMENT_ID (attachment ID)
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- MON$TRANSACTION_ID (transaction ID)
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- MON$STATEMENT_STATE (statement state)
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0: idle
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1: active
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2: cancelled
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- MON$TIMESTAMP (statement start date/time)
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- MON$SQL_TEXT (statement text, if appropriate)
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Notes:
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1) Textual descriptions of all "state" and "mode" values can be found
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in the system table RDB$TYPES
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2) For table MON$ATTACHMENTS:
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- column MON$REMOTE_PID contains non-NULL value only if the client
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library has version 2.1 or higher
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3) For table MON$STATEMENTS:
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- column MON$SQL_TEXT contains NULL for GDML statements
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- columns MON$TRANSACTION_ID and MON$TIMESTAMP contain valid values
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for active statements only
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Example(s):
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1) Retrieve IDs of all CS processes loading CPU at the moment
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SELECT MON$SERVER_PID
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FROM MON$ATTACHMENTS
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WHERE MON$ATTACHMENT_ID <> CURRENT_CONNECTION
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AND MON$ATTACHMENT_STATE = 1
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2) Retrieve information about client applications
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SELECT MON$USER, MON$REMOTE_ADDRESS, MON$REMOTE_PID, MON$TIMESTAMP
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FROM MON$ATTACHMENTS
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WHERE MON$ATTACHMENT_ID <> CURRENT_CONNECTION
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3) Get isolation level of the current transaction
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SELECT MON$ISOLATION_LEVEL
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FROM MON$TRANSACTIONS
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WHERE MON$TRANSACTION_ID = CURRENT_TRANSACTION
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4) Get statements that are currently active
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SELECT ATT.MON$USER, ATT.MON$REMOTE_ADDRESS, STMT.MON$SQL_TEXT, STMT.MON$TIMESTAMP
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FROM MON$ATTACHMENTS ATT
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JOIN MON$STATEMENTS STMT ON ATT.MON$ATTACHMENT_ID = STMT.MON$ATTACHMENT_ID
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WHERE ATT.MON$ATTACHMENT_ID <> CURRENT_CONNECTION
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AND STMT.MON$STATEMENT_STATE = 1
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