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diff --git a/src/include/core/memstore.h b/src/include/core/memstore.h
--- /dev/null
@@ -0,0 +1,535 @@
+/*
+ * SysDB - src/include/core/memstore.h
+ * Copyright (C) 2012-2015 Sebastian 'tokkee' Harl <sh@tokkee.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
+ * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
+ * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
+ * OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
+ * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef SDB_CORE_MEMSTORE_H
+#define SDB_CORE_MEMSTORE_H 1
+
+#include "sysdb.h"
+#include "core/object.h"
+#include "core/data.h"
+#include "core/store.h"
+#include "core/time.h"
+#include "parser/ast.h"
+#include "utils/strbuf.h"
+
+#include <stdbool.h>
+#include <stdio.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * sdb_memstore_t represents an in-memory store. It inherits from sdb_object_t
+ * and may safely be case to a generic object.
+ */
+struct sdb_memstore;
+typedef struct sdb_memstore sdb_memstore_t;
+#define SDB_MEMSTORE(obj) ((sdb_memstore_t *)(obj))
+
+/*
+ * sdb_memstore_obj_t represents the super-class of any stored object. It
+ * inherits from sdb_object_t and may safely be cast to a generic object to
+ * access its name.
+ */
+struct sdb_memstore_obj;
+typedef struct sdb_memstore_obj sdb_memstore_obj_t;
+
+/*
+ * Expressions represent arithmetic expressions based on stored objects and
+ * their various attributes.
+ *
+ * An expression object inherits from sdb_object_t and, thus, may safely be
+ * cast to a generic object.
+ */
+struct sdb_memstore_expr;
+typedef struct sdb_memstore_expr sdb_memstore_expr_t;
+#define SDB_MEMSTORE_EXPR(obj) ((sdb_memstore_expr_t *)(obj))
+
+/*
+ * An expression iterator iterates over the values of an iterable expression.
+ */
+struct sdb_memstore_expr_iter;
+typedef struct sdb_memstore_expr_iter sdb_memstore_expr_iter_t;
+
+/*
+ * Store matchers may be used to lookup hosts from the store based on their
+ * various attributes. Service and attribute matchers are applied to a host's
+ * services and attributes and evaluate to true if *any* service or attribute
+ * matches.
+ *
+ * A store matcher object inherits from sdb_object_t and, thus, may safely be
+ * cast to a generic object.
+ */
+struct sdb_memstore_matcher;
+typedef struct sdb_memstore_matcher sdb_memstore_matcher_t;
+#define SDB_MEMSTORE_MATCHER(obj) ((sdb_memstore_matcher_t *)(obj))
+
+/*
+ * sdb_memstore_writer:
+ * A store writer implementation that provides an in-memory object store. It
+ * expects a store object as its user-data argument.
+ */
+extern sdb_store_writer_t sdb_memstore_writer;
+
+/*
+ * sdb_memstore_reader:
+ * A store reader implementation that uses an in-memory object store. It
+ * expects a store object as its user-data argument.
+ */
+extern sdb_store_reader_t sdb_memstore_reader;
+
+/*
+ * sdb_memstore_create:
+ * Allocate a new in-memory store.
+ */
+sdb_memstore_t *
+sdb_memstore_create(void);
+
+/*
+ * sdb_memstore_host, sdb_memstore_service, sdb_memstore_metric,
+ * sdb_memstore_attribute, sdb_memstore_metric_attr:
+ * Store an object in the specified store. The hostname is expected to be
+ * canonical.
+ */
+int
+sdb_memstore_host(sdb_memstore_t *store, const char *name, sdb_time_t last_update);
+int
+sdb_memstore_service(sdb_memstore_t *store, const char *hostname, const char *name,
+ sdb_time_t last_update);
+int
+sdb_memstore_metric(sdb_memstore_t *store, const char *hostname, const char *name,
+ sdb_metric_store_t *metric_store, sdb_time_t last_update);
+int
+sdb_memstore_attribute(sdb_memstore_t *store, const char *hostname,
+ const char *key, const sdb_data_t *value, sdb_time_t last_update);
+int
+sdb_memstore_service_attr(sdb_memstore_t *store, const char *hostname,
+ const char *service, const char *key, const sdb_data_t *value,
+ sdb_time_t last_update);
+int
+sdb_memstore_metric_attr(sdb_memstore_t *store, const char *hostname,
+ const char *metric, const char *key, const sdb_data_t *value,
+ sdb_time_t last_update);
+
+/*
+ * sdb_memstore_get_host:
+ * Query the specified store for a host by its (canonicalized) name.
+ *
+ * The function increments the ref count of the host object. The caller needs
+ * to deref it when no longer using it.
+ */
+sdb_memstore_obj_t *
+sdb_memstore_get_host(sdb_memstore_t *store, const char *name);
+
+/*
+ * sdb_memstore_get_child:
+ * Retrieve a host's child object of the specified type and name. The
+ * reference count of the child object will be incremented before returning
+ * it. The caller is responsible for releasing the object once it's no longer
+ * used.
+ *
+ * Returns:
+ * - the child object on success
+ * - NULL else
+ */
+sdb_memstore_obj_t *
+sdb_memstore_get_child(sdb_memstore_obj_t *host, int type, const char *name);
+
+/*
+ * sdb_memstore_get_field:
+ * Get the value of a stored object's queryable field. The caller is
+ * responsible for freeing any dynamically allocated memory possibly stored in
+ * the returned value. If 'res' is NULL, the function will return whether the
+ * field exists.
+ *
+ * Returns:
+ * - 0 on success
+ * - a negative value else
+ */
+int
+sdb_memstore_get_field(sdb_memstore_obj_t *obj, int field, sdb_data_t *res);
+
+/*
+ * sdb_memstore_get_attr:
+ * Get the value of a stored object's attribute. The caller is responsible for
+ * freeing any dynamically allocated memory possibly stored in the returned
+ * value. If 'res' is NULL, the function will return whether the attribute
+ * exists. If specified, only attributes matching the filter will be
+ * considered.
+ *
+ * Returns:
+ * - 0 if the attribute exists
+ * - a negative value else
+ */
+int
+sdb_memstore_get_attr(sdb_memstore_obj_t *obj, const char *name, sdb_data_t *res,
+ sdb_memstore_matcher_t *filter);
+
+/*
+ * Querying a store:
+ *
+ * - Query interface: A query is a formal description of an interaction with
+ * the store. It can be used, both, for read and write access. The query is
+ * described by its abstract syntax tree (AST). The parser package provides
+ * means to parse a string (SysQL) representation of the query into an AST.
+ *
+ * - Matcher / expression interface: This low-level interface provides direct
+ * control over how to access the store. It is used by the query
+ * implementation internally and can only be used for read access.
+ */
+
+/*
+ * sdb_memstore_query_t:
+ * A parsed query readily prepared for execution.
+ */
+struct sdb_memstore_query;
+typedef struct sdb_memstore_query sdb_memstore_query_t;
+
+/*
+ * sdb_memstore_query_prepare:
+ * Prepare the query described by 'ast' for execution in a store.
+ *
+ * Returns:
+ * - a store query on success
+ * - NULL else
+ */
+sdb_memstore_query_t *
+sdb_memstore_query_prepare(sdb_ast_node_t *ast);
+
+/*
+ * sdb_memstore_query_prepare_matcher:
+ * Prepare the logical expression described by 'ast' for execution as a store
+ * matcher.
+ *
+ * Returns:
+ * - a matcher on success
+ * - NULL else
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_query_prepare_matcher(sdb_ast_node_t *ast);
+
+/*
+ * sdb_memstore_query_execute:
+ * Execute a previously prepared query in the specified store. The query
+ * result will be written to 'buf' and any errors to 'errbuf'.
+ *
+ * Returns:
+ * - the result type (to be used by the server reply)
+ * - a negative value on error
+ */
+int
+sdb_memstore_query_execute(sdb_memstore_t *store, sdb_memstore_query_t *m,
+ sdb_store_writer_t *w, sdb_object_t *wd, sdb_strbuf_t *errbuf);
+
+/*
+ * sdb_memstore_expr_create:
+ * Creates an arithmetic expression implementing the specified operator on the
+ * specified left and right operand.
+ *
+ * Returns:
+ * - an expression object on success
+ * - NULL else
+ */
+sdb_memstore_expr_t *
+sdb_memstore_expr_create(int op,
+ sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+
+/*
+ * sdb_memstore_expr_typed:
+ * Creates an expression which evaluates in the context of an object's sibling
+ * as specified by the given type.
+ *
+ * Returns:
+ * - an expression object on success
+ * - NULL else
+ */
+sdb_memstore_expr_t *
+sdb_memstore_expr_typed(int typ, sdb_memstore_expr_t *expr);
+
+/*
+ * sdb_memstore_expr_fieldvalue:
+ * Creates an expression which evaluates to the value of the specified
+ * queryable field of a stored object.
+ *
+ * Returns:
+ * - an expression object on success
+ * - NULL else
+ */
+sdb_memstore_expr_t *
+sdb_memstore_expr_fieldvalue(int field);
+
+/*
+ * sdb_memstore_expr_attrvalue:
+ * Creates an expression which evaluates to the value of the specified
+ * attribute of a stored object. Evaluates to a NULL value if the attribute
+ * does not exist.
+ *
+ * Returns:
+ * - an expression object on success
+ * - NULL else
+ */
+sdb_memstore_expr_t *
+sdb_memstore_expr_attrvalue(const char *name);
+
+/*
+ * sdb_memstore_expr_constvalue:
+ * Creates an expression which evaluates to the specified constant value.
+ *
+ * Returns:
+ * - an expression object on success
+ * - NULL else
+ */
+sdb_memstore_expr_t *
+sdb_memstore_expr_constvalue(const sdb_data_t *value);
+
+/*
+ * sdb_memstore_expr_eval:
+ * Evaluate an expression for the specified stored object and stores the
+ * result in 'res'. The result's value will be allocated dynamically if
+ * necessary and, thus, should be free'd by the caller (e.g. using
+ * sdb_data_free_datum). The object may be NULL, in which case the expression
+ * needs to evaluate to a constant value. If specified, only objects matching
+ * the filter will be used during the evaluation.
+ *
+ * Returns:
+ * - 0 on success
+ * - a negative value else
+ */
+int
+sdb_memstore_expr_eval(sdb_memstore_expr_t *expr, sdb_memstore_obj_t *obj,
+ sdb_data_t *res, sdb_memstore_matcher_t *filter);
+
+/*
+ * sdb_memstore_expr_iter:
+ * Iterate over the elements of an iterable expression. sdb_memstore_expr_iter
+ * returns NULL if the expression is not iterable (for the specified object).
+ *
+ * sdb_memstore_expr_iter_get_next returns NULL if there is no next element.
+ */
+sdb_memstore_expr_iter_t *
+sdb_memstore_expr_iter(sdb_memstore_expr_t *expr, sdb_memstore_obj_t *obj,
+ sdb_memstore_matcher_t *filter);
+void
+sdb_memstore_expr_iter_destroy(sdb_memstore_expr_iter_t *iter);
+
+bool
+sdb_memstore_expr_iter_has_next(sdb_memstore_expr_iter_t *iter);
+sdb_data_t
+sdb_memstore_expr_iter_get_next(sdb_memstore_expr_iter_t *iter);
+
+/*
+ * sdb_memstore_dis_matcher:
+ * Creates a matcher matching the disjunction (logical OR) of two matchers.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_dis_matcher(sdb_memstore_matcher_t *left, sdb_memstore_matcher_t *right);
+
+/*
+ * sdb_memstore_con_matcher:
+ * Creates a matcher matching the conjunction (logical AND) of two matchers.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_con_matcher(sdb_memstore_matcher_t *left, sdb_memstore_matcher_t *right);
+
+/*
+ * sdb_memstore_inv_matcher:
+ * Creates a matcher matching the inverse (logical NOT) of a matcher.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_inv_matcher(sdb_memstore_matcher_t *m);
+
+/*
+ * sdb_memstore_any_matcher:
+ * Creates a matcher iterating over values of the first expression (which has
+ * to be iterable). It matches if *any* of those elements match 'm'. 'm' has
+ * to be an ary operation with the left operand unset.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_any_matcher(sdb_memstore_expr_t *iter, sdb_memstore_matcher_t *m);
+
+/*
+ * sdb_memstore_all_matcher:
+ * Creates a matcher iterating over values of the first expression (which has
+ * to be iterable). It matches if *all* of those elements match 'm'. 'm' has
+ * to be an ary operation with the left operand unset.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_all_matcher(sdb_memstore_expr_t *iter, sdb_memstore_matcher_t *m);
+
+/*
+ * sdb_memstore_in_matcher:
+ * Creates a matcher which matches if the right value evaluates to an array
+ * value and the left value is included in that array. See sdb_data_inarray
+ * for more details.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_in_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+
+/*
+ * sdb_memstore_lt_matcher, sdb_memstore_le_matcher, sdb_memstore_eq_matcher,
+ * sdb_memstore_ge_matcher, sdb_memstore_gt_matcher:
+ * Create conditional matchers comparing the values of two expressions. The
+ * matcher matches if the left expression compres less than, less or equal
+ * than, equal to, not equal to, greater or equal than, or greater than the
+ * right expression.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_lt_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+sdb_memstore_matcher_t *
+sdb_memstore_le_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+sdb_memstore_matcher_t *
+sdb_memstore_eq_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+sdb_memstore_matcher_t *
+sdb_memstore_ne_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+sdb_memstore_matcher_t *
+sdb_memstore_ge_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+sdb_memstore_matcher_t *
+sdb_memstore_gt_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+
+/*
+ * sdb_memstore_regex_matcher:
+ * Creates a matcher which matches the string value the left expression
+ * evaluates to against the regular expression the right expression evaluates
+ * to. The right expression may either be a constant value regular expression
+ * or string or a dynamic value evaluating to a string. In the latter case,
+ * the string is compiled to a regex every time the matcher is executed.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_regex_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+
+/*
+ * sdb_memstore_nregex_matcher:
+ * Creates a regex matcher just like sdb_memstore_regex_matcher except that it
+ * matches in case the regular expression does not match.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_nregex_matcher(sdb_memstore_expr_t *left, sdb_memstore_expr_t *right);
+
+/*
+ * sdb_memstore_isnull_matcher:
+ * Creates a matcher matching NULL values.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_isnull_matcher(sdb_memstore_expr_t *expr);
+
+/*
+ * sdb_memstore_istrue_matcher, sdb_memstore_isfalse_matcher:
+ * Creates a matcher matching boolean values.
+ */
+sdb_memstore_matcher_t *
+sdb_memstore_istrue_matcher(sdb_memstore_expr_t *expr);
+sdb_memstore_matcher_t *
+sdb_memstore_isfalse_matcher(sdb_memstore_expr_t *expr);
+
+/*
+ * sdb_memstore_matcher_matches:
+ * Check whether the specified matcher matches the specified store object. If
+ * specified, the filter will be used to preselect objects for further
+ * evaluation. It is applied to any object that's used during the evaluation
+ * of the matcher. Only those objects matching the filter will be considered.
+ *
+ * Note that the filter is applied to all object types (hosts, service,
+ * metric, attribute). Thus, any object-specific matchers are mostly unsuited
+ * for this purpose and, if used, may result in unexpected behavior.
+ *
+ * Returns:
+ * - 1 if the object matches
+ * - 0 else
+ */
+int
+sdb_memstore_matcher_matches(sdb_memstore_matcher_t *m, sdb_memstore_obj_t *obj,
+ sdb_memstore_matcher_t *filter);
+
+/*
+ * sdb_memstore_matcher_op_cb:
+ * Callback constructing a matcher operator.
+ */
+typedef sdb_memstore_matcher_t *(*sdb_memstore_matcher_op_cb)
+ (sdb_memstore_expr_t *, sdb_memstore_expr_t *);
+
+/*
+ * sdb_memstore_lookup_cb:
+ * Lookup callback. It is called for each matching object when looking up data
+ * in the store passing on the lookup filter and the specified user-data. The
+ * lookup aborts early if the callback returns non-zero.
+ */
+typedef int (*sdb_memstore_lookup_cb)(sdb_memstore_obj_t *obj,
+ sdb_memstore_matcher_t *filter, void *user_data);
+
+/*
+ * sdb_memstore_scan:
+ * Look up objects of the specified type in the specified store. The specified
+ * callback function is called for each object in the store matching 'm'. The
+ * function performs a full scan of all stored objects. If specified, the
+ * filter will be used to preselect objects for further evaluation. See the
+ * description of 'sdb_memstore_matcher_matches' for details.
+ *
+ * Returns:
+ * - 0 on success
+ * - a negative value else
+ */
+int
+sdb_memstore_scan(sdb_memstore_t *store, int type,
+ sdb_memstore_matcher_t *m, sdb_memstore_matcher_t *filter,
+ sdb_memstore_lookup_cb cb, void *user_data);
+
+/*
+ * sdb_memstore_emit:
+ * Send a single object to the specified store writer. Attributes or any child
+ * objects are not included. Use sdb_memstore_emit_full() to emit a full
+ * (filtered) object.
+ *
+ * Returns:
+ * - 0 on success
+ * - a negative value else
+ */
+int
+sdb_memstore_emit(sdb_memstore_obj_t *obj, sdb_store_writer_t *w, sdb_object_t *wd);
+
+/*
+ * sdb_memstore_emit_full:
+ * Send a single object and its attributes and all children to the specified
+ * store writer. The filter, if specified, is applied to each attribute and
+ * child object. Only matching objects will be emitted.
+ *
+ * Returns:
+ * - 0 on success
+ * - a negative value else
+ */
+int
+sdb_memstore_emit_full(sdb_memstore_obj_t *obj, sdb_memstore_matcher_t *filter,
+ sdb_store_writer_t *w, sdb_object_t *wd);
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* ! SDB_CORE_MEMSTORE_H */
+
+/* vim: set tw=78 sw=4 ts=4 noexpandtab : */
+