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/*! @page devdoc-dhandle-lifecycle Data Handle Lifecycle
A WiredTiger Data Handle (dhandle) is a generic representation of any named
data source. This representation contains information such as its name,
how many references there are to it, individual data source statistics and
what type of underlying data object it is.
WiredTiger maintains all dhandles in a global dhandle list accessed
from the connection. Multiple sessions access this list, which is
protected by a R/W lock. Each session also maintains a session
dhandle cache which is a cache of dhandles a session has operated
upon. The entries in the cache are references into the global dhandle
list.
@section dhandle-creation dhandle creation
When a cursor in a session attempts to access a WiredTiger table
that has not been accessed before, the dhandle for the table will
neither be in the session's dhandle cache nor in the connection's
global dhandle list. A new dhandle will have to be created for this
table. The cursor trying to access a table first attempts to find
the dhandle in the session dhandle cache. When it doesn't find the
dhandle in the session cache, it searches in the global dhandle
list while holding the read lock. When it doesn't find the dhandle
there, it creates a dhandle for this table and puts it in the global
dhandle list while holding the write lock on the global dhandle
list. The cursor operation then puts a reference to the dhandle in
the session's dhandle cache.
There are two relevant reference counters in the dhandle structure,
\c session_ref and \c session_inuse. \c session_ref counts the
number of session dhandle cache lists that this contain dhandle.
\c session_inuse is a count of cursors opened and operating on this
dhandle. Both these counters are incremented by this session as the
cursor attempts to use this dhandle. When the operation completes
and the cursor is closed, the \c session_inuse is decremented. The
dhandle reference is not removed immediately from the session dhandle
cache. If this session accesses the same table again in the near
future, having the dhandle reference already in the session dhandle
cache is a performance optimization.
@section session-cache-sweep dhandle session cache sweep
Dhandle cache sweep is the only way a cleanup is performed on the
session' dhandle cache list. The references to the dhandles that have not
been accessed by this session in a long time are removed from the
cache. Since a session is single-threaded, a session's dhandle cache
can only be altered by that session alone.
Each time a session accesses a dhandle, it checks if enough
time has elapsed to do a session cache sweep for that session.
As it walks the session dhandle cache list, it notices if any dhandle
on its list has been marked dead (idle too long). If it has, the
session removes that dhandle from its list and decrements the
\c session_ref count.
Since accessing a dhandle involves walking the session dhandle cache
list anyway, cache cleanup is piggy-backed on this operation.
@section dhandle-sweep sweep-server dhandle sweep
WiredTiger maintains a sweep server in the background for the cleanup of the
global dhandle list. The sweep server periodically (\c close_scan_interval)
revisits the dhandles in the global list and if the dhandles are not being used,
i.e., the \c session_inuse count is 0, it assigns the current time as the time
of death for the dhandle, if not already done before.
If a dhandle has already got a time of death set for it in a previous iteration
of the dhandle sweep and the dhandle has stayed not in use, the sweep server
compares the time of death with the current time to check if the dhandle has
remained idle for the configured idle time (\c close_idle_time). If the dhandle
has remained idle, the sweep server closes the associated btree contained in the
dhandle and releases some of the resources for that dhandle. It also marks
the dhandle as dead so that the next time a session with a reference walks its
own cache list, it will see the handle marked dead and remove it from the
session's dhandle cache list (see above).
The sweep server then checks for whether or not any session is referencing this
dhandle, i.e., if a session's dhandle cache still contains a reference to this
dhandle. If a dhandle stays referenced by at least one session, i.e., the
\c session_ref count is not 0, the dhandle can't be removed from the global
list. If this dhandle is not referenced by any session, i.e., \c session_ref
count is 0, the sweep server removes the dhandle from the global dhandle list
and frees any remaining resources associated with it. The removal of the dhandle
from the global list hence completes this dhandle's lifecycle. Any future access
of the associated table would need to start by creating the dhandle again.
Note: The sweep server's scan interval and a dhandle's close idle time can be
configured using \c file_manager configuration settings of the connection
handle.
*/
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