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Diffstat (limited to 'iredis/data/commands/bitcount.md')
| -rw-r--r-- | iredis/data/commands/bitcount.md | 47 |
1 files changed, 28 insertions, 19 deletions
diff --git a/iredis/data/commands/bitcount.md b/iredis/data/commands/bitcount.md index 680aae1..95bd3a3 100644 --- a/iredis/data/commands/bitcount.md +++ b/iredis/data/commands/bitcount.md @@ -1,8 +1,8 @@ Count the number of set bits (population counting) in a string. -By default all the bytes contained in the string are examined. It is possible to -specify the counting operation only in an interval passing the additional -arguments _start_ and _end_. +By default all the bytes contained in the string are examined. +It is possible to specify the counting operation only in an interval passing the +additional arguments _start_ and _end_. Like for the `GETRANGE` command start and end can contain negative values in order to index bytes starting from the end of the string, where -1 is the last @@ -10,6 +10,11 @@ byte, -2 is the penultimate, and so forth. Non-existent keys are treated as empty strings, so the command will return zero. +By default, the additional arguments _start_ and _end_ specify a byte index. +We can use an additional argument `BIT` to specify a bit index. +So 0 is the first bit, 1 is the second bit, and so forth. +For negative values, -1 is the last bit, -2 is the penultimate, and so forth. + @return @integer-reply @@ -23,44 +28,48 @@ SET mykey "foobar" BITCOUNT mykey BITCOUNT mykey 0 0 BITCOUNT mykey 1 1 +BITCOUNT mykey 1 1 BYTE +BITCOUNT mykey 5 30 BIT ``` ## Pattern: real-time metrics using bitmaps Bitmaps are a very space-efficient representation of certain kinds of -information. One example is a Web application that needs the history of user -visits, so that for instance it is possible to determine what users are good -targets of beta features. +information. +One example is a Web application that needs the history of user visits, so that +for instance it is possible to determine what users are good targets of beta +features. Using the `SETBIT` command this is trivial to accomplish, identifying every day -with a small progressive integer. For instance day 0 is the first day the -application was put online, day 1 the next day, and so forth. +with a small progressive integer. +For instance day 0 is the first day the application was put online, day 1 the +next day, and so forth. -Every time a user performs a page view, the application can register that in the -current day the user visited the web site using the `SETBIT` command setting the -bit corresponding to the current day. +Every time a user performs a page view, the application can register that in +the current day the user visited the web site using the `SETBIT` command setting +the bit corresponding to the current day. Later it will be trivial to know the number of single days the user visited the web site simply calling the `BITCOUNT` command against the bitmap. -A similar pattern where user IDs are used instead of days is described in the -article called "[Fast easy realtime metrics using Redis +A similar pattern where user IDs are used instead of days is described +in the article called "[Fast easy realtime metrics using Redis bitmaps][hbgc212fermurb]". -[hbgc212fermurb]: - http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps +[hbgc212fermurb]: http://blog.getspool.com/2011/11/29/fast-easy-realtime-metrics-using-redis-bitmaps ## Performance considerations In the above example of counting days, even after 10 years the application is online we still have just `365*10` bits of data per user, that is just 456 bytes -per user. With this amount of data `BITCOUNT` is still as fast as any other O(1) -Redis command like `GET` or `INCR`. +per user. +With this amount of data `BITCOUNT` is still as fast as any other O(1) Redis +command like `GET` or `INCR`. When the bitmap is big, there are two alternatives: -- Taking a separated key that is incremented every time the bitmap is modified. +* Taking a separated key that is incremented every time the bitmap is modified. This can be very efficient and atomic using a small Redis Lua script. -- Running the bitmap incrementally using the `BITCOUNT` _start_ and _end_ +* Running the bitmap incrementally using the `BITCOUNT` _start_ and _end_ optional parameters, accumulating the results client-side, and optionally caching the result into a key. |