path: root/libnetdata/dictionary/README.md

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# Dictionaries

Netdata dictionaries associate a `name` with a `value`:

- A `name` can be any string.
- A `value` can be anything.

Such a pair of a `name` and a `value` consists of an `item` or an `entry` in the dictionary.

Dictionaries provide an interface to:

- **Add** an item to the dictionary
- **Get** an item from the dictionary (provided its `name`)
- **Delete** an item from the dictionary (provided its `name`)
- **Traverse** the list of items in the dictionary

Dictionaries are **ordered**, meaning that the order they have been added is preserved while traversing them. The caller may reverse this order by passing the flag `DICTIONARY_FLAG_ADD_IN_FRONT` when creating the dictionary.

Dictionaries guarantee **uniqueness** of all items added to them, meaning that only one item with a given name can exist in the dictionary at any given time.

Dictionaries are extremely fast in all operations. They are indexing the keys with `JudyHS` (or `AVL` when `libJudy` is not available) and they utilize a double-linked-list for the traversal operations. Deletion is the most expensive operation, usually somewhat slower than insertion.

## Memory management

Dictionaries come with 2 memory management options:

- **Clone** (copy) the name and/or the value to memory allocated by the dictionary.
- **Link** the name and/or the value, without allocating any memory about them.

In **clone** mode, the dictionary guarantees that all operations on the dictionary items will automatically take care of the memory used by the name and/or the value. In case the value is an object that needs to have user allocated memory, the following callback functions can be registered:

   1.`dictionary_register_insert_callback()` that will be called just after the insertion of an item to the dictionary, or after the replacement of the value of a dictionary item (but while the dictionary is write-locked - if locking is enabled).
   2. `dictionary_register_delete_callback()` that will be called just prior to the deletion of an item from the dictionary, or prior to the replacement of the value of a dictionary item (but while the dictionary is write-locked - if locking is enabled).
   3. `dictionary_register_conflict_callback()` that will be called when `DICTIONARY_FLAG_DONT_OVERWRITE_VALUE` is set and another value is attempted to be inserted for the same key. 

In **link** mode, the name and/or the value are just linked to the dictionary item, and it is the user's responsibility to free the memory they use after an item is deleted from the dictionary or when the dictionary is destroyed.

By default, **clone** mode is used for both the name and the value.

To use **link** mode for names, add `DICTIONARY_FLAG_NAME_LINK_DONT_CLONE` to the flags when creating the dictionary.

To use **link** mode for values, add `DICTIONARY_FLAG_VALUE_LINK_DONT_CLONE` to the flags when creating the dictionary.

## Locks

The dictionary allows both **single-threaded** operation (no locks - faster) and **multi-threaded** operation utilizing a read-write lock.

The default is **multi-threaded**. To enable **single-threaded** add `DICTIONARY_FLAG_SINGLE_THREADED` to the flags when creating the dictionary.

## Hash table operations

The dictionary supports the following operations supported by the hash table:

- `dictionary_set()` to add an item to the dictionary, or change its value.
- `dictionary_get()` to get an item from the dictionary.
- `dictionary_del()` to delete an item from the dictionary.

## Creation and destruction

Use `dictionary_create()` to create a dictionary.

Use `dictionary_destroy()` to destroy a dictionary. When destroyed, a dictionary frees all the memory it has allocated on its own. This can be complemented by the registration of a deletion callback function that can be called upon deletion of each item in the dictionary, which may free additional resources. 

### dictionary_set()

This call is used to:

- **add** an item to the dictionary.
- **reset** the value of an existing item in the dictionary.

If **resetting** is not desired, add `DICTIONARY_FLAG_DONT_OVERWRITE_VALUE` to the flags when creating the dictionary. In this case, `dictionary_set()` will return the value of the original item found in the dictionary instead of resetting it and the value passed to the call will be ignored. Optionally a conflict callback function can be registered, to manipulate (probably merge or extend) the original value, based on the new value attempted to be added to the dictionary.

For **multi-threaded** operation, the `dictionary_set()` calls get an exclusive write lock on the dictionary.

The format is:

```c
value = dictionary_set(dict, name, value, value_len);
```

Where:

* `dict` is a pointer to the dictionary previously created.
* `name` is a pointer to a string to be used as the key of this item. The name must not be `NULL` and must not be an empty string `""`.
* `value` is a pointer to the value associated with this item. In **clone** mode, if `value` is `NULL`, a new memory allocation will be made of `value_len` size and will be initialized to zero.
* `value_len` is the size of the `value` data. If `value_len` is zero, no allocation will be done and the dictionary item will permanently have the `NULL` value.

> **IMPORTANT**<br/>There is also an **unsafe** version (without locks) of this call. This is to be used when traversing the dictionary in write mode. It should never be called without an active lock on the dictionary, which can only be acquired while traversing.

### dictionary_get()

This call is used to get the value of an item, given its name. It utilizes the `JudyHS` hash table for making the lookup.

For **multi-threaded** operation, the `dictionary_get()` call gets a shared read lock on the dictionary.

In clone mode, the value returned is not guaranteed to be valid, as any other thread may delete the item from the dictionary at any time. To ensure the value will be available, use `dictionary_get_and_acquire_item()`, which uses a reference counter to defer deletes until the item is released.

The format is:

```c
value = dictionary_get(dict, name);
```

Where:

* `dict` is a pointer to the dictionary previously created.
* `name` is a pointer to a string to be used as the key of this item. The name must not be `NULL` and must not be an empty string `""`.

> **IMPORTANT**<br/>There is also an **unsafe** version (without locks) of this call. This is to be used when traversing the dictionary. It should never be called without an active lock on the dictionary, which can only be acquired while traversing.

### dictionary_del()

This call is used to delete an item from the dictionary, given its name.

If there is a deletion callback registered to the dictionary (`dictionary_register_delete_callback()`), it is called prior to the actual deletion of the item. 

For **multi-threaded** operation, the `dictionary_del()` calls get an exclusive write lock on the dictionary.

The format is:

```c
value = dictionary_del(dict, name);
```

Where:

* `dict` is a pointer to the dictionary previously created.
* `name` is a pointer to a string to be used as the key of this item. The name must not be `NULL` and must not be an empty string `""`.

> **IMPORTANT**<br/>There is also an **unsafe** version (without locks) of this call. This is to be used when traversing the dictionary, to delete the current item. It should never be called without an active lock on the dictionary, which can only be acquired while traversing.

### dictionary_get_and_acquire_item()

This call can be used the search and get a dictionary item, while ensuring that it will be available for use, until `dictionary_acquired_item_release()` is called.

This call **does not return the value** of the dictionary item. It returns an internal pointer to a structure that maintains the reference counter used to protect the actual value. To get the value of the item (the same value as returned by `dictionary_get()`), the function `dictionary_acquired_item_value()` has to be called.

Example:

```c
// create the dictionary
DICTIONARY *dict = dictionary_create(DICTIONARY_FLAGS_NONE);

// add an item to it
dictionary_set(dict, "name", "value", 6);

// find the item we added and acquire it
void *item = dictionary_get_and_acquire_item(dict, "name");

// extract its value
char *value = (char *)dictionary_acquired_item_value(dict, item);

// now value points to the string "value"
printf("I got value = '%s'\n", value);

// release the item, so that it can deleted
dictionary_acquired_item_release(dict, item);

// destroy the dictionary
dictionary_destroy(dict);
```

When items are acquired, a reference counter is maintained to keep track of how many users exist for it. If an item with a non-zero number of users is deleted, it is removed from the index, it can be added again to the index (without conflict), and although it exists in the linked-list, it is not offered during traversal. Garbage collection to actually delete the item happens every time a write-locked dictionary is unlocked (just before the unlock) and items are deleted only if no users are using them.

If any item is still acquired when the dictionary is destroyed, the destruction of the dictionary is also deferred until all the acquired items are released. When the dictionary is destroyed like that, all operations on the dictionary fail (traversals do not traverse, insertions do not insert, deletions do not delete, searches do not find any items, etc). Once the last item in the dictionary is released, the dictionary is automatically destroyed too.

## Traversal

Dictionaries offer 3 ways to traverse the entire dictionary:

- **walkthrough**, implemented by setting a callback function to be called for every item.
- **sorted walkthrough**, which first sorts the dictionary and then call a callback function for every item.
- **foreach**, a way to traverse the dictionary with a for-next loop.

All these methods are available in **read** or **write** mode. In **read** mode only lookups are allowed to the dictionary. In **write** lookups but also insertions and deletions are allowed.

While traversing the dictionary with any of these methods, all calls to the dictionary have to use the `_unsafe` versions of the function calls, otherwise deadlocks may arise.

> **IMPORTANT**<br/>The dictionary itself does not check to ensure that a user is actually using the right lock mode (read or write) while traversing the dictionary for each of the unsafe calls.

### walkthrough (callback)

There are 4 calls:

- `dictionary_walkthrough_read()` and `dictionary_sorted_walkthrough_read()` that acquire a shared read lock, and they call a callback function for every item of the dictionary. The callback function may use the unsafe versions of the `dictionary_get()` calls to lookup other items in the dictionary, but it should not attempt to add or remove items to/from the dictionary.
- `dictionary_walkthrough_write()` and `dictionary_sorted_walkthrough_write()` that acquire an exclusive write lock, and they call a callback function for every item of the dictionary. This is to be used when items need to be added to or removed from the dictionary. The `write` versions can be used to delete any or all the items from the dictionary, including the currently working one. For the `sorted` version, all items in the dictionary maintain a reference counter, so all deletions are deferred until the sorted walkthrough finishes.** 

The non sorted versions traverse the items in the same order they have been added to the dictionary (or the reverse order if the flag `DICTIONARY_FLAG_ADD_IN_FRONT` is set during dictionary creation). The sorted versions sort alphabetically the items based on their name, and then they traverse them in the sorted order.

The callback function returns an `int`. If this value is negative, traversal of the dictionary is stopped immediately and the negative value is returned to the caller. If the returned value of all callback calls is zero or positive, the walkthrough functions return the sum of the return values of all callbacks. So, if you are just interested to know how many items fall into some condition, write a callback function that returns 1 when the item satisfies that condition and 0 when it does not and the walkthrough function will return how many tested positive.

### foreach (for-next loop)

The following is a snippet of such a loop:

```c
MY_ITEM *item;
dfe_start_read(dict, item) {
   printf("hey, I got an item named '%s' with value ptr %08X", item_name, item);
}
dfe_done(item);
```

The `item` parameter gives the name of the pointer to be used while iterating the items. Any name is accepted.

The `item_name` is a variable that is automatically created, by concatenating whatever is given as `item` and `_name`. So, if you call `dfe_start_read(dict, myvar)`, the name will be `myvar_name`.

Both `dfe_start_read(dict, item)` and `dfe_done(item)` are together inside a `do { ... } while(0)` loop, so that the following will work:

```c
MY_ITEM *item;

if(x = 1)
    // do {
    dfe_start_read(dict, item)
        printf("hey, I got an item named '%s' with value ptr %08X", item_name, item);
    dfe_done(item);
    // } while(0);
else
    something else;
```

In the above, the `if(x == 1)` condition will work as expected. It will do the foreach loop when x is 1, otherwise it will run `something else`.

There are 2 versions of `dfe_start`:

- `dfe_start_read()` that acquires a shared read lock to the dictionary.
- `dfe_start_write()` that acquires an exclusive write lock to the dictionary.

While in the loop, depending on the read or write versions of `dfe_start`, the caller may lookup or manipulate the dictionary using the unsafe functions. The rules are the same with the unsorted walkthrough callback functions.

PS: DFE is Dictionary For Each.

## special multi-threaded lockless case

Since the dictionary uses a hash table and a double linked list, if the contract between 2 threads is for one to use the hash table functions only (`set`, `get` - but no `del`) and the other to use the traversal ones only, the dictionary allows concurrent use without locks.

This is currently used in statsd:

- the data collection thread uses only `get` and `set`. It never uses `del`. New items are added at the front of the linked list (`DICTIONARY_FLAG_ADD_IN_FRONT`).
- the flushing thread is only traversing the dictionary up to the point it last traversed it (it uses a flag for that to know where it stopped last time). It never uses `get`, `set` or `del`.