readme: add 'developer guide'/library reference with code examples

PettitWesley · edsiper · commit 14b7eb79e7ec · 2020-01-17T10:29:34.000+10:00
Signed-off-by: Wesley Pettit &lt;wppttt@amazon.com&gt;
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
@@ -2,6 +2,10 @@
 
 We build Open Source software and we invite everyone to join us and contribute. So if you are interested into participate, please refer to the guidelines below.
 
+## Developer Guide
+
+[Developer Guide with code examples](DEVELOPER_GUIDE.md).
+
 ## GIT Repositories
 
 All code changes and submissions happens on [Github](http://github.com), that means that to start contributing you should clone the target repository, perform local changes and then do a Pull Request. For more details about the workflow we suggest you check the following documents:
@@ -75,7 +79,7 @@ the proper way is to perform the variable definitions on top:
 int flb_something(int a, int b)
 {
     int ret;
-    
+
     if (a > 10) {
         return 1;
     }
@@ -146,24 +150,24 @@ When you commit your local changes in your repository (before to push to Github)
 
    Common components prefix are:
 
-   - utils: 
+   - utils:
    - pack:
    - sds:
    - http_client:
 
-   As you can see prefixes are basically the file name of the source code file under [src](https://github.com/fluent/fluent-bit/tree/master/src) directory without the file prefix <u>flb_</u>. 
+   As you can see prefixes are basically the file name of the source code file under [src](https://github.com/fluent/fluent-bit/tree/master/src) directory without the file prefix <u>flb_</u>.
 
    When committing changes to code that's related to some plugins, the commit subject must be prefixed with the name of the plugin being changed, e.g:
 
    - in_stdin:
-   - out_http: 
+   - out_http:
    - out_kafka:
 
    please refer to the [plugins](https://github.com/fluent/fluent-bit/tree/master/plugins) directory as a reference
 
 - One single commit **must not** include changes to files that are different from the component specified in the subject, e.g: If you are extending flb_utils.c file, the git patch should not touch any other file than flb_utils.c or flb_utils.h.
 
-- One single commit **must not** include multiple prefixes to specify different areas being touched. 
+- One single commit **must not** include multiple prefixes to specify different areas being touched.
 
  - The subject of the commit **must not** be longer than 80 characters.
 
diff --git a/DEVELOPER_GUIDE.md b/DEVELOPER_GUIDE.md
@@ -0,0 +1,330 @@
+## Beginners Guide to Contributing to Fluent Bit
+
+Assuming you have some basic knowledge of C, this guide should help you understand how to make code
+changes to Fluent Bit.
+
+### Table of Contents
+- [Libraries](#libraries)
+    - [Memory Management](#memory-management)
+    - [Strings](#strings)
+    - [HTTP Client](#http-client)
+    - [Linked Lists](#linked-lists)
+    - [Message Pack](#message-pack)
+- [Plugin API](#plugin-api)
+    - [Input](#input)
+    - [Filter](#filter)
+    - [Output](#output)
+- [Testing](#testing)
+- [Need more help?](#need-more-help)
+
+### Libraries
+
+Most external libraries are embedded in the project in the [/lib](/lib) folder. To keep its footprint low and make cross-platform builds simple, Fluent Bit attempts keep its dependency graph small.
+
+The external library you are mostly likely to interact with is [msgpack](https://github.com/msgpack/msgpack-c).
+
+For crypto, Fluent Bit uses [mbedtls](https://github.com/ARMmbed/mbedtls).
+
+#### Memory Management
+
+When you write Fluent Bit code, you will use Fluent Bit's versions of the standard C functions for working with memory:
+- [`flb_alloc()`](include/fluent-bit/flb_mem.h) - equivalent to `malloc`, allocates memory.
+- [`flb_calloc()`](include/fluent-bit/flb_mem.h)  - equivalent to `calloc`, allocates memory and initializes it to zero.
+- [`flb_realloc()`](include/fluent-bit/flb_mem.h) - equivalent to `realloc`.
+- [`flb_free()`](include/fluent-bit/flb_mem.h) - equivalent to `free`, releases allocated memory.
+
+Note that many types have a specialized create and destroy function. For example,
+[`flb_sds_create()` and `flb_sds_destroy()`](include/fluent-bit/flb_sds.h).
+
+#### Strings
+
+Fluent Bit has a stripped down version of the popular [SDS](https://github.com/antirez/sds) string library. See [flb_sds.h](include/fluent-bit/flb_sds.h) for the API.
+
+In general, you should use SDS strings in any string processing code. SDS strings are fully compatible with any C function that accepts a null-terminated sequence of characters; to understand how they work, see the [explanation on Github](https://github.com/antirez/sds#how-sds-strings-work).
+
+#### HTTP Client
+
+Fluent Bit has its own network connection library. The key types and functions are defined in the following header files:
+- [flb_upstream.h](include/fluent-bit/flb_upstream.h)
+- [flb_http_client.h](include/fluent-bit/flb_http_client.h)
+- [flb_io.h](include/fluent-bit/flb_io.h)
+
+The following code demonstrates making an HTTP request in Fluent Bit:
+
+```c
+#include <fluent-bit/flb_upstream.h>
+#include <fluent-bit/flb_io.h>
+#include <fluent-bit/flb_http_client.h>
+#include <fluent-bit/flb_info.h>
+#include <fluent-bit/flb_config.h>
+
+#define HOST  "127.0.0.1"
+#define PORT  80
+
+static flb_sds_t make_request(struct flb_config *config)
+{
+    struct flb_upstream *upstream;
+    struct flb_http_client *client;
+    size_t b_sent;
+    int ret;
+    struct flb_upstream_conn *u_conn;
+    flb_sds_t resp;
+
+    upstream = flb_upstream_create(config, HOST, PORT, FLB_IO_TCP, NULL);
+    if (!upstream) {
+        flb_error("[example] connection initialization error");
+        return -1;
+    }
+
+    u_conn = flb_upstream_conn_get(upstream);
+    if (!u_conn) {
+        flb_error("[example] connection initialization error");
+        flb_upstream_destroy(upstream);
+        return -1;
+    }
+
+    /* Compose HTTP Client request */
+    client = flb_http_client(u_conn,
+                             FLB_HTTP_GET, metadata_path,
+                             NULL, 0,
+                             FLB_FILTER_AWS_IMDS_V2_HOST, 80,
+                             NULL, 0);
+
+    if (!client) {
+        flb_error("[example] count not create http client");
+        flb_upstream_conn_release(u_conn);
+        flb_upstream_destroy(upstream);
+        return -1;
+    }
+
+    if (ret != 0 || client->resp.status != 200) {
+        if (client->resp.payload_size > 0) {
+            flb_debug("[example] Request failed and returned: \n%s",
+                      client->resp.payload);
+        }
+        flb_http_client_destroy(client);
+        flb_upstream_conn_release(u_conn);
+        flb_upstream_destroy(upstream);
+        return -1;
+    }
+
+    data = flb_sds_create_len(client->resp.payload,
+                                   client->resp.payload_size);
+
+    flb_http_client_destroy(client);
+    flb_upstream_conn_release(u_conn);
+    flb_upstream_destroy(upstream);
+
+    return resp;
+}
+```
+
+An `flb_upstream` structure represents a host/endpoint that you want to call. Normally, you'd store this structure somewhere so that it can be re-used. An `flb_upstream_conn` represents a connection to that host for a single HTTP request. The connection structure should not be used for more than one request.
+
+#### Linked Lists
+
+Fluent Bit contains a library for constructing linked lists- [mk_list](lib/monkey/include/monkey/mk_core/mk_list.h). The type stores data as a circular linked list.
+
+The [`mk_list.h`](lib/monkey/include/monkey/mk_core/mk_list.h) header file contains several macros and functions for use with the lists. The example below shows how to create a list, iterate through it, and delete an element.
+
+```c
+#include <monkey/mk_core/mk_list.h>
+#include <fluent-bit/flb_info.h>
+
+struct item {
+    char some_data;
+
+    struct mk_list _head;
+};
+
+static int example()
+{
+    struct mk_list *tmp;
+    struct mk_list *head;
+    struct mk_list items;
+    int i;
+    char characters[] = "abcdefghijk";
+    struct item *an_item;
+
+    /* construct a list */
+    mk_list_init(&items);
+
+    for (i = 0; i < strlen(characters); i++) {
+        an_item = flb_malloc(sizeof(struct item));
+        if (!an_item) {
+            flb_errno();
+            return -1;
+        }
+        an_item->some_data = characters[i];
+
+        mk_list_add(&an_item->_head, &items);
+    }
+
+    /* iterate through the list */
+    flb_info("Iterating through list");
+    mk_list_foreach_safe(head, tmp, &items) {
+        an_item = mk_list_entry(head, struct item, _head);
+        flb_info("list item data value: %c", an_item->some_data);
+    }
+
+    /* remove an item */
+    mk_list_foreach_safe(head, tmp, &items) {
+        an_item = mk_list_entry(head, struct item, _head);
+        if (an_item->some_data == 'b') {
+            mk_list_del(&an_item->_head);
+            flb_free(an_item);
+        }
+    }
+}
+```
+
+#### Message Pack
+
+Fluent Bit uses [msgpack](https://msgpack.org/index.html) to internally store data. If you write code for Fluent Bit, it is almost certain that you will interact with msgpack.
+
+Fluent Bit embeds the [msgpack-c](https://github.com/msgpack/msgpack-c) library. The example below shows manipulating message pack to add a new key-value pair to a record. In Fluent Bit, the [filter_record_modifier](plugins/filter_record_modifier) plugin adds or deletes keys from records. See its code for more.
+
+```
+#define A_NEW_KEY        "key"
+#define A_NEW_KEY_LEN    3
+#define A_NEW_VALUE      "value"
+#define A_NEW_VALUE_LEN  5
+
+static int cb_filter(const void *data, size_t bytes,
+                     const char *tag, int tag_len,
+                     void **out_buf, size_t *out_size,
+                     struct flb_filter_instance *f_ins,
+                     void *context,
+                     struct flb_config *config)
+{
+    (void) f_ins;
+    (void) config;
+    size_t off = 0;
+    int i = 0;
+    int ret;
+    struct flb_time tm;
+    int total_records;
+    int new_keys = 1;
+    msgpack_sbuffer tmp_sbuf;
+    msgpack_packer tmp_pck;
+    msgpack_unpacked result;
+    msgpack_object  *obj;
+    msgpack_object_kv *kv;
+
+    /* Create temporary msgpack buffer */
+    msgpack_sbuffer_init(&tmp_sbuf);
+    msgpack_packer_init(&tmp_pck, &tmp_sbuf, msgpack_sbuffer_write);
+
+    /* Iterate over each item */
+    msgpack_unpacked_init(&result);
+    while (msgpack_unpack_next(&result, data, bytes, &off) == MSGPACK_UNPACK_SUCCESS) {
+        /*
+         * Each record is a msgpack array [timestamp, map] of the
+         * timestamp and record map. We 'unpack' each record, and then re-pack
+         * it with the new fields added.
+         */
+
+        if (result.data.type != MSGPACK_OBJECT_ARRAY) {
+            continue;
+        }
+
+        /* unpack the array of [timestamp, map] */
+        flb_time_pop_from_msgpack(&tm, &result, &obj);
+
+        /* obj should now be the record map */
+        if (obj->type != MSGPACK_OBJECT_MAP) {
+            continue;
+        }
+
+        /* re-pack the array into a new buffer */
+        msgpack_pack_array(&tmp_pck, 2);
+        flb_time_append_to_msgpack(&tm, &tmp_pck, 0);
+
+        /* new record map size is old size + the new keys we will add */
+        total_records = obj->via.map.size + new_keys;
+        msgpack_pack_map(&tmp_pck, total_records);
+
+        /* iterate through the old record map and add it to the new buffer */
+        kv = obj->via.map.ptr;
+        for(i=0; i < obj->via.map.size; i++) {
+            msgpack_pack_object(&tmp_pck, (kv+i)->key);
+            msgpack_pack_object(&tmp_pck, (kv+i)->val);
+        }
+
+        /* append new keys */
+        msgpack_pack_str(&tmp_pck, A_NEW_KEY_LEN);
+        msgpack_pack_str_body(&tmp_pck, A_NEW_KEY, A_NEW_KEY_LEN);
+        msgpack_pack_str(&tmp_pck, A_NEW_VALUE_LEN);
+        msgpack_pack_str_body(&tmp_pck, A_NEW_VALUE, A_NEW_VALUE_LEN);
+
+    }
+    msgpack_unpacked_destroy(&result);
+
+    /* link new buffers */
+    *out_buf  = tmp_sbuf.data;
+    *out_size = tmp_sbuf.size;
+    return FLB_FILTER_MODIFIED;
+```
+
+Please also check out the message pack examples on the [msgpack-c GitHub repo](https://github.com/msgpack/msgpack-c).
+
+### Plugin API
+
+Each plugin is a shared object which is [loaded into Fluent Bit](https://github.com/fluent/fluent-bit/blob/1.3/src/flb_plugin.c#L70) using dlopen and dlsym.
+
+#### Input
+
+The input plugin structure is defined in [flb_input.h](https://github.com/fluent/fluent-bit/blob/master/include/fluent-bit/flb_input.h#L62). There are a number of functions which a plugin can implement, most only implement `cb_init`, `cb_collect`, and `cb_exit`.
+
+The [`"dummy"` input plugin](plugins/in_dummy) very simple and is an excellent example to review to understand more.
+
+#### Filter
+
+The structure for filter plugins is defined in [flb_filter.h](https://github.com/fluent/fluent-bit/blob/master/include/fluent-bit/flb_filter.h#L44). Each plugin must implement `cb_init`, `cb_filter`, and `cb_exit`.
+
+The [filter_record_modifier](plugins/filter_record_modifier) is a good example of a filter plugin.
+
+Note that filter plugins can not asynchronously make HTTP requests. If your plugin needs to make a request, add the following code when you initialize your `flb_upstream`:
+
+```c
+/* Remove async flag from upstream */
+upstream->flags &= ~(FLB_IO_ASYNC);
+```
+
+#### Output
+
+Output plugins are defined in [flb_output.h](https://github.com/fluent/fluent-bit/blob/master/include/fluent-bit/flb_output.h#L57). Each plugin must implement `cb_init`, `cb_flush`, and `cb_exit`.
+
+The [stdout plugin](plugins/out_stdout) is very simple; review its code to understand how output plugins work.
+
+### Testing
+
+During development, you can build Fluent Bit as follows:
+
+```
+cd build
+cmake -DFLB_DEV=On ../
+make
+```
+Note that Fluent Bit uses Cmake 3 and on some systems you may need to invoke it as `cmake3`.
+
+To enable the unit tests run:
+```
+cmake -DFLB_DEV=On -DFLB_TESTS_RUNTIME=On -DFLB_TESTS_INTERNAL=On ../
+make
+```
+
+Internal tests are for the internal libraries of Fluent Bit. Runtime tests are for the plugins.
+
+You can run the unit tests with `make test`, however, this is inconvenient in practice. Each test file will create an executable in the `build/bin` directory which you can run directly. For example, if you want to run the SDS tests, you can invoke them as follows:
+
+```
+$ ./bin/flb-it-sds
+Test sds_usage...                               [   OK   ]
+Test sds_printf...                              [   OK   ]
+SUCCESS: All unit tests have passed.
+```
+
+### Need more help?
+
+The best way to learn how Fluent Bit code works is to read it. If you need help understanding the code, reach out to the community, or open a PR with changes that are a work in progress.
