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mqtree: Index tree for MQTT topic filters

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mqtree is an Erlang NIF implementation of N-ary tree to keep MQTT topic filters for efficient matching.

System requirements

To compile mqtree you need:

  • GNU Make.
  • GCC.
  • Erlang/OTP 17.5 or higher.

Compiling

$ git clone https://github.com/processone/mqtree.git
$ cd mqtree
$ make

API

new/0

-spec new() -> tree().

Creates new tree. The tree is mutable just like ETS, so there is no need to keep its updated version between calls. The created tree gets destroyed when it's garbage collected.

Complexity: O(1).

NOTE: a registered tree (see register/2) is not a subject for garbage collection until unregister/1 is called explicitly.

insert/2

-spec insert(Tree :: tree(), Filter :: iodata()) -> ok.

Inserts Filter into Tree and increases its reference counter. The reference counter is increased every time when the same filter is inserted into the tree. The reference counter is decreased when the filter is deleted, see delete/2.

Complexity: O(H) where H is the number of slashes (/) in Filter.

NOTE: no checks are performed on the filter being inserted: it's up to the caller to check if the filter conforms to the MQTT specification.

delete/2

-spec delete(Tree :: tree(), Filter :: iodata()) -> ok.

Deletes Filter from Tree and decreases its reference counter. Nothing is done if the filter is not found in the tree.

Complexity: O(H) where H is the number of slashes (/) in Filter.

NOTE: no checks are performed on the filter being deleted: it's up to the caller to check if the filter conforms to the MQTT specification.

match/2

-spec match(Tree :: tree(), Path :: iodata()) -> [binary()].

Finds filters in Tree matching Path according to the MQTT specification.

Complexity: O(2^H) worst case, where H is the number of slashes (/) in Path. Note that the worst case complexity is only achieved when an attacker forces to store in the tree a massive amount of filters containing + meta-symbol. The obvious protection is to restrict the filter depth. Another approach is to make filter "deduplication" during subscription registration, e.g. filters a/+, +/b and +/+ should be "merged" into single +/+.

NOTE: no checks are performed on the path being matched: it's up to the caller to check if the path conforms to the MQTT specification.

NOTE: any path starting with $ won't match filters starting with + or #. This is in accordance with the MQTT specification.

refc/2

-spec refc(Tree :: tree(), Filter :: iodata()) -> non_neg_intger().

Returns the reference counter of Filter in Tree. In particular, zero (0) is returned if the filter is not found in the tree.

Complexity: O(H) where H is the number of slashes (/) in Filter.

NOTE: no checks are performed on the filter being searched: it's up to the caller to check if the filter conforms to the MQTT specification.

clear/1

-spec clear(Tree :: tree()) -> ok.

Deletes all filters from Tree.

Complexity: O(N) where N is the number of filters in the tree.

size/1

-spec size(Tree :: tree()) -> non_neg_integer().

Returns the size of Tree. That is, the number of filters in the tree (irrespective of their reference counters).

Complexity: O(N) where N is the number of filters in the tree.

is_empty/1

-spec is_empty(Tree :: tree()) -> boolean().

Returns true if Tree holds no filters. Returns false otherwise.

Complexity: O(1).

register/2

-spec register(RegName :: atom(), Tree :: tree()) -> ok.

Associates RegName with Tree. The tree is then available via call to whereis/1. Fails with badarg exception if:

  • RegName is already in use (even by the tree being registered)
  • RegName is atom undefined
  • Either RegName or Tree has invalid type

It is safe to register already registered tree to another name. In this case the old name will be freed automatically.

Complexity: O(1).

NOTE: a registered tree is not a subject for garbage collection. You must call unregister/1 explicitly if you want the tree to be freed by garbage collector.

unregister/1

-spec unregister(RegName :: atom()) -> ok.

Removes the registered name RegName associated with a tree. Fails with badarg exception if RegName is not a registered name.

Complexity: O(1).

whereis/1

-spec whereis(RegName :: atom()) -> Tree :: tree() | undefined.

Returns Tree with registered name RegName. Returns undefined otherwise.

Complexity: O(1).