From: Sebastian Harl <sh@tokkee.org>
Date: Wed, 5 Aug 2015 08:38:48 +0000 (+0200)
Subject: core: Add support for reader/query plugins.
X-Git-Tag: sysdb-0.8.0~57
X-Git-Url: https://git.tokkee.org/?p=sysdb.git;a=commitdiff_plain;h=182112ccd9852672b39ff958ab0c605c9ef856ad

core: Add support for reader/query plugins.

A reader plugin can be used to query a data-store based on a query described
by an AST. The plugin module provides a generic function for this purpose.

At the moment, only a single reader plugin can be used at a time. Supporting
multiple readers requires a good merge strategy. The challenge is to handle
filters and query conditions correctly which may evaluate differently after
two stores have been merged.
---

diff --git a/src/core/plugin.c b/src/core/plugin.c
index 3ba98df..26b1408 100644
--- a/src/core/plugin.c
+++ b/src/core/plugin.c
@@ -106,6 +106,14 @@ typedef struct {
 } writer_t;
 #define WRITER(obj) ((writer_t *)(obj))
 
+typedef struct {
+	callback_t super; /* cb_callback will always be NULL */
+#define r_user_data super.cb_user_data
+#define r_ctx super.cb_ctx
+	sdb_store_reader_t impl;
+} reader_t;
+#define READER(obj) ((reader_t *)(obj))
+
 /*
  * private variables
  */
@@ -127,6 +135,7 @@ static sdb_llist_t      *shutdown_list = NULL;
 static sdb_llist_t      *log_list = NULL;
 static sdb_llist_t      *ts_fetcher_list = NULL;
 static sdb_llist_t      *writer_list = NULL;
+static sdb_llist_t      *reader_list = NULL;
 
 static struct {
 	const char   *type;
@@ -140,6 +149,7 @@ static struct {
 	{ "log",                &log_list },
 	{ "timeseries fetcher", &ts_fetcher_list },
 	{ "store writer",       &writer_list },
+	{ "store reader",       &reader_list },
 };
 
 /*
@@ -436,6 +446,53 @@ static sdb_type_t writer_type = {
 	plugin_writer_destroy
 };
 
+static int
+plugin_reader_init(sdb_object_t *obj, va_list ap)
+{
+	sdb_store_reader_t *impl = va_arg(ap, sdb_store_reader_t *);
+	sdb_object_t       *ud   = va_arg(ap, sdb_object_t *);
+
+	assert(impl);
+
+	if ((! impl->prepare_query) || (! impl->execute_query)) {
+		sdb_log(SDB_LOG_ERR, "core: store reader callback '%s' "
+				"does not fully implement the reader interface.",
+				obj->name);
+		return -1;
+	}
+	if (sdb_llist_search_by_name(reader_list, obj->name)) {
+		sdb_log(SDB_LOG_WARNING, "core: store reader callback '%s' "
+				"has already been registered. Ignoring newly "
+				"registered version.", obj->name);
+		return -1;
+	}
+
+	/* ctx may be NULL if the callback was not registered by a plugin */
+
+	READER(obj)->impl = *impl;
+	READER(obj)->r_ctx  = ctx_get();
+	sdb_object_ref(SDB_OBJ(READER(obj)->r_ctx));
+
+	sdb_object_ref(ud);
+	READER(obj)->r_user_data = ud;
+	return 0;
+} /* plugin_reader_init */
+
+static void
+plugin_reader_destroy(sdb_object_t *obj)
+{
+	assert(obj);
+	sdb_object_deref(READER(obj)->r_user_data);
+	sdb_object_deref(SDB_OBJ(READER(obj)->r_ctx));
+} /* plugin_reader_destroy */
+
+static sdb_type_t reader_type = {
+	sizeof(reader_t),
+
+	plugin_reader_init,
+	plugin_reader_destroy
+};
+
 static int
 module_init(const char *name, lt_dlhandle lh, sdb_plugin_info_t *info)
 {
@@ -867,6 +924,41 @@ sdb_plugin_register_writer(const char *name,
 	return 0;
 } /* sdb_store_register_writer */
 
+int
+sdb_plugin_register_reader(const char *name,
+		sdb_store_reader_t *reader, sdb_object_t *user_data)
+{
+	char cb_name[1024];
+	sdb_object_t *obj;
+
+	if ((! name) || (! reader))
+		return -1;
+
+	if (! reader_list)
+		reader_list = sdb_llist_create();
+	if (! reader_list)
+		return -1;
+
+	plugin_get_name(name, cb_name, sizeof(cb_name));
+
+	obj = sdb_object_create(cb_name, reader_type,
+			reader, user_data);
+	if (! obj)
+		return -1;
+
+	if (sdb_llist_append(reader_list, obj)) {
+		sdb_object_deref(obj);
+		return -1;
+	}
+
+	/* pass control to the list */
+	sdb_object_deref(obj);
+
+	sdb_log(SDB_LOG_INFO, "core: Registered store reader callback '%s'.",
+			cb_name);
+	return 0;
+} /* sdb_plugin_register_reader */
+
 void
 sdb_plugin_unregister_all(void)
 {
@@ -1330,6 +1422,40 @@ sdb_plugin_fetch_timeseries(const char *type, const char *id,
 	return ts;
 } /* sdb_plugin_fetch_timeseries */
 
+int
+sdb_plugin_query(sdb_ast_node_t *ast, sdb_strbuf_t *buf, sdb_strbuf_t *errbuf)
+{
+	size_t n = sdb_llist_len(reader_list);
+	reader_t *reader;
+	sdb_object_t *q;
+	int status = 0;
+
+	if (! ast)
+		return 0;
+
+	if (n != 1) {
+		char *msg = (n > 0)
+			? "Cannot execute query: multiple readers not supported"
+			: "Cannot execute query: no readers registered";
+		sdb_strbuf_sprintf(errbuf, "%s", msg);
+		sdb_log(SDB_LOG_ERR, "core: %s", msg);
+		return -1;
+	}
+
+	reader = READER(sdb_llist_get(reader_list, 0));
+	assert(reader);
+
+	q = reader->impl.prepare_query(ast, errbuf, reader->r_user_data);
+	if (q)
+		status = reader->impl.execute_query(q, buf, errbuf, reader->r_user_data);
+	else
+		status = -1;
+
+	sdb_object_deref(SDB_OBJ(q));
+	sdb_object_deref(SDB_OBJ(reader));
+	return status;
+} /* sdb_plugin_query */
+
 int
 sdb_plugin_store_host(const char *name, sdb_time_t last_update)
 {
diff --git a/src/core/store.c b/src/core/store.c
index f672c12..2d32e0d 100644
--- a/src/core/store.c
+++ b/src/core/store.c
@@ -831,6 +831,31 @@ static sdb_store_writer_t store_writer = {
 	store_attribute, store_service_attr, store_metric_attr,
 };
 
+/*
+ * TODO: let prepare and execute accept a store object as their user_data
+ * object
+ */
+
+static sdb_object_t *
+prepare_query(sdb_ast_node_t *ast,
+		sdb_strbuf_t __attribute__((unused)) *errbuf,
+		sdb_object_t __attribute__((unused)) *user_data)
+{
+	return SDB_OBJ(sdb_store_query_prepare(ast));
+} /* prepare_query */
+
+static int
+execute_query(sdb_object_t *q,
+		sdb_strbuf_t *buf, sdb_strbuf_t *errbuf,
+		sdb_object_t __attribute__((unused)) *user_data)
+{
+	return sdb_store_query_execute(QUERY(q), buf, errbuf);
+} /* execute_query */
+
+static sdb_store_reader_t store_reader = {
+	prepare_query, execute_query,
+};
+
 /*
  * public API
  */
diff --git a/src/include/core/plugin.h b/src/include/core/plugin.h
index 1155a95..6b5a9f4 100644
--- a/src/include/core/plugin.h
+++ b/src/include/core/plugin.h
@@ -267,7 +267,7 @@ sdb_plugin_register_ts_fetcher(const char *name,
  *
  * Arguments:
  *  - user_data: If specified, this will be passed on to each call of the
- *    callback. The function will take ownership of the object, that is,
+ *    callbacks. The function will take ownership of the object, that is,
  *    increment the reference count by one. In case the caller does not longer
  *    use the object for other purposes, it should thus deref it.
  */
@@ -275,6 +275,22 @@ int
 sdb_plugin_register_writer(const char *name,
 		sdb_store_writer_t *writer, sdb_object_t *user_data);
 
+/*
+ * sdb_plugin_register_reader:
+ * Register a "reader" implementation for querying the store. It is invalid to
+ * register an incomplete reader which does not implement all of the reader
+ * interface.
+ *
+ * Arguments:
+ *  - user_data: If specified, this will be passed on to each call of the
+ *    callbacks. The function will take ownership of the object, that is,
+ *    increment the reference count by one. In case the caller does not longer
+ *    use the object for other purposes, it should thus deref it.
+ */
+int
+sdb_plugin_register_reader(const char *name,
+		sdb_store_reader_t *reader, sdb_object_t *user_data);
+
 /*
  * sdb_plugin_unregister_all:
  * Unregister all registered plugins and destruct their user-data objects.
@@ -420,6 +436,18 @@ sdb_timeseries_t *
 sdb_plugin_fetch_timeseries(const char *type, const char *id,
 		sdb_timeseries_opts_t *opts);
 
+/*
+ * sdb_plugin_query:
+ * Query the store using the query specified by 'ast'. The result will be
+ * written to 'buf' and any errors will be written to 'errbuf'.
+ *
+ * Returns:
+ *  - 0 on success
+ *  - a negative value else
+ */
+int
+sdb_plugin_query(sdb_ast_node_t *ast, sdb_strbuf_t *buf, sdb_strbuf_t *errbuf);
+
 /*
  * sdb_plugin_store_host, sdb_plugin_store_service, sdb_plugin_store_metric,
  * sdb_plugin_store_attribute, sdb_plugin_store_service_attribute,
diff --git a/src/include/core/store.h b/src/include/core/store.h
index 289bc2f..e851dd9 100644
--- a/src/include/core/store.h
+++ b/src/include/core/store.h
@@ -229,6 +229,32 @@ typedef struct {
 			sdb_object_t *user_data);
 } sdb_store_writer_t;
 
+/*
+ * A store reader describes the interface to query a store implementation.
+ */
+typedef struct {
+	/*
+	 * prepare_query:
+	 * Prepare the query described by 'ast' for execution.
+	 */
+	sdb_object_t *(*prepare_query)(sdb_ast_node_t *ast,
+			sdb_strbuf_t *errbuf, sdb_object_t *user_data);
+
+	/*
+	 * execute_query:
+	 * Execute a previously prepared query. The callback may expect that only
+	 * queries prepared by its respective prepare callback will be passed to
+	 * this function.
+	 *
+	 * TODO: Instead of letting the executor write directly to a string buffer
+	 *       (which cannot easily be merged with other results), let it hand
+	 *       all objects to a store-writer.
+	 */
+	int (*execute_query)(sdb_object_t *q,
+			sdb_strbuf_t *buf, sdb_strbuf_t *errbuf,
+			sdb_object_t *user_data);
+} sdb_store_reader_t;
+
 /*
  * sdb_store_init:
  * Initialize the store sub-system. This function has to be called before