# otp.ReadCache

### *class* ReadCache(cache_name=None, db=utils.adaptive_to_default, symbol=utils.adaptive, tick_type=utils.adaptive, start=utils.adaptive, end=utils.adaptive, read_mode='automatic', update_cache=True, otq_params=None, create_cache_query='', schema=None, \*\*kwargs)

Bases: [`Source`](../source/root.md#onetick.py.Source)

Make cached query

Cache is initialized on the first read attempt.

* **Parameters:**
  * **cache_name** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) -- Name of cache for a query.
  * **symbol** (str, list of str, [`Source`](../source/root.md#onetick.py.Source), [`query`](../misc/query.md#onetick.py.query), [`eval query`](../misc/eval.md#onetick.py.eval)) -- Symbol(s) from which data should be taken.
  * **db** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) -- Database to use for tick generation.
  * **tick_type** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) -- Tick type.
    Default: ANY.
  * **start** ([`otp.datetime`](../datetime/dt.md#onetick.py.datetime)) -- Start time for tick generation. By default the start time of the query will be used.
  * **end** ([`otp.datetime`](../datetime/dt.md#onetick.py.datetime)) -- End time for tick generation. By default the end time of the query will be used.
  * **read_mode** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) -- 

    Mode of querying cache. One of these:
    * `cache_only` - only cached results are returned and queries are not performed.
    * `query_only` - the query is run irrespective of whether the result is already available in the cache.
    * `automatic` (default) - perform the query if the data is not found in the cache.
  * **update_cache** ([*bool*](https://docs.python.org/3/library/functions.html#bool)) -- If set to `True`, updates the cached data if `read_mode=query_only` or if `read_mode=automatic` and
    the result data not found in the cache. Otherwise, the cache remains unchanged.
  * **otq_params** ([*dict*](https://docs.python.org/3/library/stdtypes.html#dict)) -- OTQ parameters to override `otq_params` that are passed during creation of cache.
    Query result will be cached separately for each unique pair of OTQ parameters.
  * **create_cache_query** ([*str*](https://docs.python.org/3/library/stdtypes.html#str)) -- If a cache with the given name is not present, the query provided in this param will be invoked,
    which should contain CREATE_CACHE EP to create the corresponding cache.
  * **schema** (*Optional* *,* [*dict*](https://docs.python.org/3/library/stdtypes.html#dict)) -- Dictionary of columns names with their types.
  * **kwargs** -- Deprecated. Use `schema` instead.
    Dictionary of columns names with their types.

### Examples

Simple cache read:

```pycon
>>> def query_func():
...    return otp.DataSource("DEMO_L1", tick_type="TRD", symbols="AAPL")
>>> otp.create_cache(
...    cache_name="some_cache", query=query_func, tick_type="TRD", db="DEMO_L1",
... )
>>> src = otp.ReadCache("some_cache")
>>> otp.run(src)  
```

Make cache query for specific time interval:

```pycon
>>> src = otp.ReadCache(
...    "some_cache",
...    start=otp.datetime(2024, 1, 1, 12, 30),
...    end=otp.datetime(2024, 1, 2, 18, 0),
... )
>>> otp.run(src)  
```

Override or set otq_params for one query:

```pycon
>>> src = otp.ReadCache(
...    "some_cache",
...    otq_params={"some_param": "test_value"},
... )
>>> otp.run(src)  
```

#### SEE ALSO
**READ_CACHE** OneTick event processor
<br/>
[`onetick.py.create_cache()`](../functions/cache/create_cache.md#onetick.py.create_cache)
<br/>