Database interactions
*********************
Database object
===============
.. py:class:: rocksdb.DB
.. py:method:: __init__(db_name, Options opts, read_only=False)
:param unicode db_name: Name of the database to open
:param opts: Options for this specific database
:type opts: :py:class:`rocksdb.Options`
:param bool read_only: If ``True`` the database is opened read-only.
All DB calls which modify data will raise an
Exception.
.. py:method:: put(key, value, sync=False, disable_wal=False)
Set the database entry for "key" to "value".
:param bytes key: Name for this entry
:param bytes value: Data for this entry
:param bool sync:
If ``True``, the write will be flushed from the operating system
buffer cache (by calling WritableFile::Sync()) before the write
is considered complete. If this flag is true, writes will be
slower.
If this flag is ``False``, and the machine crashes, some recent
writes may be lost. Note that if it is just the process that
crashes (i.e., the machine does not reboot), no writes will be
lost even if ``sync == False``.
In other words, a DB write with ``sync == False`` has similar
crash semantics as the "write()" system call. A DB write
with ``sync == True`` has similar crash semantics to a "write()"
system call followed by "fdatasync()".
:param bool disable_wal:
If ``True``, writes will not first go to the write ahead log,
and the write may got lost after a crash.
.. py:method:: delete(key, sync=False, disable_wal=False)
Remove the database entry for "key".
:param bytes key: Name to delete
:param sync: See :py:meth:`rocksdb.DB.put`
:param disable_wal: See :py:meth:`rocksdb.DB.put`
:raises rocksdb.errors.NotFound: If the key did not exists
.. py:method:: merge(key, value, sync=False, disable_wal=False)
Merge the database entry for "key" with "value".
The semantics of this operation is determined by the user provided
merge_operator when opening DB.
See :py:meth:`rocksdb.DB.put` for the parameters
:raises:
:py:exc:`rocksdb.errors.NotSupported` if this is called and
no :py:attr:`rocksdb.Options.merge_operator` was set at creation
.. py:method:: write(batch, sync=False, disable_wal=False)
Apply the specified updates to the database.
:param rocksdb.WriteBatch batch: Batch to apply
:param sync: See :py:meth:`rocksdb.DB.put`
:param disable_wal: See :py:meth:`rocksdb.DB.put`
.. py:method:: get(key, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
:param bytes key: Name to get
:param bool verify_checksums:
If ``True``, all data read from underlying storage will be
verified against corresponding checksums.
:param bool fill_cache:
Should the "data block", "index block" or "filter block"
read for this iteration be cached in memory?
Callers may wish to set this field to ``False`` for bulk scans.
:param snapshot:
If not ``None``, read as of the supplied snapshot
(which must belong to the DB that is being read and which must
not have been released). Is it ``None`` a implicit snapshot of the
state at the beginning of this read operation is used
:type snapshot: :py:class:`rocksdb.Snapshot`
:param string read_tier:
Specify if this read request should process data that ALREADY
resides on a particular cache. If the required data is not
found at the specified cache,
then :py:exc:`rocksdb.errors.Incomplete` is raised.
| Use ``all`` if a fetch from disk is allowed.
| Use ``cache`` if only data from cache is allowed.
:returns: ``None`` if not found, else the value for this key
.. py:method:: multi_get(keys, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
:param keys: Keys to fetch
:type keys: list of bytes
For the other params see :py:meth:`rocksdb.DB.get`
:returns:
A ``dict`` where the value is either ``bytes`` or ``None`` if not found
:raises: If the fetch for a single key fails
.. note::
keys will not be "de-duplicated".
Duplicate keys will return duplicate values in order.
.. py:method:: key_may_exist(key, fetch=False, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
If the key definitely does not exist in the database, then this method
returns ``False``, else ``True``. If the caller wants to obtain value
when the key is found in memory, fetch should be set to ``True``.
This check is potentially lighter-weight than invoking DB::get().
One way to make this lighter weight is to avoid doing any IOs.
:param bytes key: Key to check
:param bool fetch: Obtain also the value if found
For the other params see :py:meth:`rocksdb.DB.get`
:returns:
* ``(True, None)`` if key is found but value not in memory
* ``(True, None)`` if key is found and ``fetch=False``
* ``(True, <data>)`` if key is found and value in memory and ``fetch=True``
* ``(False, None)`` if key is not found
.. py:method:: iterkeys(fetch=False, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
Iterate over the keys
For other params see :py:meth:`rocksdb.DB.get`
:returns:
A iterator object which is not valid yet.
Call first one of the seek methods of the iterator to position it
:rtype: :py:class:`rocksdb.BaseIterator`
.. py:method:: itervalues(fetch=False, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
Iterate over the values
For other params see :py:meth:`rocksdb.DB.get`
:returns:
A iterator object which is not valid yet.
Call first one of the seek methods of the iterator to position it
:rtype: :py:class:`rocksdb.BaseIterator`
.. py:method:: iteritems(fetch=False, verify_checksums=False, fill_cache=True, snapshot=None, read_tier="all")
Iterate over the items
For other params see :py:meth:`rocksdb.DB.get`
:returns:
A iterator object which is not valid yet.
Call first one of the seek methods of the iterator to position it
:rtype: :py:class:`rocksdb.BaseIterator`
.. py:method:: snapshot()
Return a handle to the current DB state.
Iterators created with this handle will all observe a stable snapshot
of the current DB state.
:rtype: :py:class:`rocksdb.Snapshot`
.. py:method:: get_property(prop)
DB implementations can export properties about their state
via this method. If "property" is a valid property understood by this
DB implementation, a byte string with its value is returned.
Otherwise ``None``
Valid property names include:
* ``b"rocksdb.num-files-at-level<N>"``: return the number of files at level <N>,
where <N> is an ASCII representation of a level number (e.g. "0").
* ``b"rocksdb.stats"``: returns a multi-line byte string that describes statistics
about the internal operation of the DB.
* ``b"rocksdb.sstables"``: returns a multi-line byte string that describes all
of the sstables that make up the db contents.
* ``b"rocksdb.num-immutable-mem-table"``: Number of immutable mem tables.
* ``b"rocksdb.mem-table-flush-pending"``: Returns ``1`` if mem table flush is pending, otherwise ``0``.
* ``b"rocksdb.compaction-pending"``: Returns ``1`` if a compaction is pending, otherweise ``0``.
* ``b"rocksdb.background-errors"``: Returns accumulated background errors encountered.
* ``b"rocksdb.cur-size-active-mem-table"``: Returns current size of the active memtable.
.. py:method:: get_live_files_metadata()
Returns a list of all table files.
It returns a list of dict's were each dict has the following keys.
``name``
Name of the file
``level``
Level at which this file resides
``size``
File size in bytes
``smallestkey``
Smallest user defined key in the file
``largestkey``
Largest user defined key in the file
``smallest_seqno``
smallest seqno in file
``largest_seqno``
largest seqno in file
.. py:method:: compact_range(begin=None, end=None, reduce_level=False, target_level=-1)
Compact the underlying storage for the key range [begin,end].
The actual compaction interval might be superset of [begin, end].
In particular, deleted and overwritten versions are discarded,
and the data is rearranged to reduce the cost of operations
needed to access the data.
This operation should typically only be invoked by users who understand
the underlying implementation.
``begin == None`` is treated as a key before all keys in the database.
``end == None`` is treated as a key after all keys in the database.
Therefore the following call will compact the entire database: ``db.compact_range()``.
Note that after the entire database is compacted, all data are pushed
down to the last level containing any data. If the total data size
after compaction is reduced, that level might not be appropriate for
hosting all the files. In this case, client could set reduce_level
to ``True``, to move the files back to the minimum level capable of holding
the data set or a given level (specified by non-negative target_level).
:param bytes begin: Key where to start compaction.
If ``None`` start at the beginning of the database.
:param bytes end: Key where to end compaction.
If ``None`` end at the last key of the database.
:param bool reduce_level: If ``True`` allow rocksdb to move the data to
another level, if the current is not big enouth.
If ``False`` you may end with a bigger level
than configured.
:param int target_level: Level where to push the the range to compact.
.. py:attribute:: options
Returns the associated :py:class:`rocksdb.Options` instance.
.. note::
Changes to this object have no effect anymore.
Consider this as read-only
Iterator
========
.. py:class:: rocksdb.BaseIterator
Base class for all iterators in this module. After creation a iterator is
invalid. Call one of the seek methods first before starting iteration
.. py:method:: seek_to_first()
Position at the first key in the source
.. py:method:: seek_to_last()
Position at the last key in the source
.. py:method:: seek(key)
:param bytes key: Position at the first key in the source that at or past
Methods to support the python iterator protocol
.. py:method:: __iter__()
.. py:method:: __next__()
.. py:method:: __reversed__()
Snapshot
========
.. py:class:: rocksdb.Snapshot
Opaque handler for a single Snapshot.
Snapshot is released if nobody holds a reference on it.
Retrieved via :py:meth:`rocksdb.DB.snapshot`
WriteBatch
==========
.. py:class:: rocksdb.WriteBatch
WriteBatch holds a collection of updates to apply atomically to a DB.
The updates are applied in the order in which they are added
to the WriteBatch. For example, the value of "key" will be "v3"
after the following batch is written::
batch = rocksdb.WriteBatch()
batch.put(b"key", b"v1")
batch.delete(b"key")
batch.put(b"key", b"v2")
batch.put(b"key", b"v3")
.. py:method:: __init__(data=None)
Creates a WriteBatch.
:param bytes data:
A serialized version of a previous WriteBatch. As retrieved
from a previous .data() call. If ``None`` a empty WriteBatch is
generated
.. py:method:: put(key, value)
Store the mapping "key->value" in the database.
:param bytes key: Name of the entry to store
:param bytes value: Data of this entry
.. py:method:: merge(key, value)
Merge "value" with the existing value of "key" in the database.
:param bytes key: Name of the entry to merge
:param bytes value: Data to merge
.. py:method:: delete(key)
If the database contains a mapping for "key", erase it. Else do nothing.
:param bytes key: Key to erase
.. py:method:: clear()
Clear all updates buffered in this batch.
.. py:method:: data()
Retrieve the serialized version of this batch.
:rtype: ``bytes``
.. py:method:: count()
Returns the number of updates in the batch
:rtype: int
Errors
======
.. py:exception:: rocksdb.errors.NotFound
.. py:exception:: rocksdb.errors.Corruption
.. py:exception:: rocksdb.errors.NotSupported
.. py:exception:: rocksdb.errors.InvalidArgument
.. py:exception:: rocksdb.errors.RocksIOError
.. py:exception:: rocksdb.errors.MergeInProgress
.. py:exception:: rocksdb.errors.Incomplete