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+If `key` already exists and is a string, this command appends the `value` at the
+end of the string. If `key` does not exist it is created and set as an empty
+string, so `APPEND` will be similar to `SET` in this special case.
+
+@return
+
+@integer-reply: the length of the string after the append operation.
+
+@examples
+
+```cli
+EXISTS mykey
+APPEND mykey "Hello"
+APPEND mykey " World"
+GET mykey
+```
+
+## Pattern: Time series
+
+The `APPEND` command can be used to create a very compact representation of a
+list of fixed-size samples, usually referred as _time series_. Every time a new
+sample arrives we can store it using the command
+
+```
+APPEND timeseries "fixed-size sample"
+```
+
+Accessing individual elements in the time series is not hard:
+
+- `STRLEN` can be used in order to obtain the number of samples.
+- `GETRANGE` allows for random access of elements. If our time series have
+  associated time information we can easily implement a binary search to get
+  range combining `GETRANGE` with the Lua scripting engine available in Redis
+  2.6.
+- `SETRANGE` can be used to overwrite an existing time series.
+
+The limitation of this pattern is that we are forced into an append-only mode of
+operation, there is no way to cut the time series to a given size easily because
+Redis currently lacks a command able to trim string objects. However the space
+efficiency of time series stored in this way is remarkable.
+
+Hint: it is possible to switch to a different key based on the current Unix
+time, in this way it is possible to have just a relatively small amount of
+samples per key, to avoid dealing with very big keys, and to make this pattern
+more friendly to be distributed across many Redis instances.
+
+An example sampling the temperature of a sensor using fixed-size strings (using
+a binary format is better in real implementations).
+
+```cli
+APPEND ts "0043"
+APPEND ts "0035"
+GETRANGE ts 0 3
+GETRANGE ts 4 7
+```