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+// Copyright (C) 2011-2021 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef LOG_DBGLVLS_H
+#define LOG_DBGLVLS_H
+
+/// \file
+///
+/// When a message is logged with DEBUG severity, the debug level associated
+/// with the message is also specified.  This debug level is a number
+/// ranging from 0 to 99; the idea is that the higher the debug level, the
+/// more detailed the message.
+///
+/// If debug messages are being logged, the logging system allows them to be
+/// filtered by debug level - only messages logged with a level equal to or
+/// less than the set debug level will be output.  (For example, if the
+/// filter is set to 30, only debug messages logged with levels in the range
+/// 0 to 30 will be output; messages logged with levels 31 to 99 will be
+/// suppressed.)
+///
+/// Levels of 30 or below are reserved for debug messages that are most
+/// likely to be useful for an administrator. Levels 31 to 99 are for use by
+/// someone familiar with the code. "Useful for an administrator" is,
+/// admittedly, a subjective term: it is loosely defined as messages helping
+/// someone diagnose a problem that they could solve without needing to dive
+/// into the code.  So it covers things like start-up steps and configuration
+/// messages.
+///
+/// In practice, this means that levels of 30 and below are most-likely to
+/// be used by the top-level programs, and 31 and above by the various
+/// libraries.
+///
+/// This file defines a set of standard debug levels for use across all loggers.
+/// In this way users can have some expectation of what will be output when
+/// enabling debugging.  Symbols are prefixed DBGLVL so as not to clash with
+/// DBG_ symbols in the various modules.
+
+namespace isc {
+namespace log {
+
+/// Process startup/shutdown debug messages.  Note that these are _debug_
+/// messages, as other messages related to startup and shutdown may be output
+/// with another severity.  For example, when the authoritative server starts
+/// up, the "server started" message could be output at a severity of INFO.
+/// "Server starting" and messages indicating the stages in startup should be
+/// debug messages output at this severity.
+extern const int DBGLVL_START_SHUT;
+
+/// This debug level is reserved for logging the exchange of messages/commands
+/// between processes, including configuration messages.
+extern const int DBGLVL_COMMAND;
+
+/// This debug level is reserved for logging the details of packet handling, such
+/// as dropping the packet for various reasons.
+extern const int DBGLVL_PKT_HANDLING;
+
+/// If the commands have associated data, this level is when they are printed.
+/// This includes configuration messages.
+extern const int DBGLVL_COMMAND_DATA;
+
+// The following constants are suggested values for common operations.
+// Depending on the exact nature of the code, modules may or may not use these
+// levels.
+
+/// Trace basic operations.
+extern const int DBGLVL_TRACE_BASIC;
+
+/// Trace data associated with the basic operations.
+extern const int DBGLVL_TRACE_BASIC_DATA;
+
+/// Trace detailed operations.
+extern const int DBGLVL_TRACE_DETAIL;
+
+/// Trace data associated with detailed operations.
+extern const int DBGLVL_TRACE_DETAIL_DATA;
+
+}   // log namespace
+}   // isc namespace
+
+#endif // LOG_DBGLVLS_H