ARLINGTON -- In the American League West, Edwin Diaz stands above all other closers.
Diaz is a prime reason why the Mariners are off to a good start in a division in which four of five teams have winning records. The other four teams have at least a tinge of mystery surrounding their closer as the first month of the season comes to an end.
Being a Major League closer can be a tenuous situation, and here's how the five AL West teams stand after four weeks.
Angels
Who's the closer? Keynan Middleton
How is it working out? Middleton has done an excellent job since being given save opportunities, going 6-for-6 in those situations and giving the Angels some stability. He has a 1.93 ERA and a 1.14 WHIP, and opponents are hitting just .224 off him. Middleton's fastball has been averaging 96.1 mph with a slider that can be wicked if he commands it well.

How secure is he? Manager Mike Scioscia has not officially named Middleton as the closer. Richard Parker started the season as the Halos' closer, but he was removed from the role when he struggled early. Middleton is still trying to establish himself, and he doesn't have an extended track record, so he could be on a short leash, too.
Who's next in line? Cam Bedrosian locked down the Angels' win over the Astros on Tuesday night. Los Angeles acquired Jim Johnson from the Braves this past offseason, and the veteran has 176 career saves, including 22 last year. The Halos just called up right-hander Justin Anderson, who throws 98 mph. He could have a future in the role.
Astros
Who's the closer? Ken Giles
How is it working out? Strangely. manager AJ Hinch said Giles is the closer, but Hinch has also used Chris Devenski and Brad Peacock at times. The skipper said things can vary depending on matchups and situations. Another issue is the Astros are so good, they don't often leave their relievers in save situations.

How secure is he? Hinch seems to be trying to keep the pressure off Giles, who had 34 saves last season for Houston, but then struggled in the playoffs and was pushed aside at the end of the World Series. He has pitched in just nine innings in 10 games, but he has a 0.67 WHIP and a 2.00 ERA. Giles has also been dealing with some back tightness, although that hasn't been an issue lately.
Who's next in line? Actually, Devenski and Giles each have two saves. Peacock also has a save. The Astros have options, and Hinch seems to be exploring them while keeping Giles nominally in the role.
Athletics
Who's the closer: Blake Treinen
How is it working out? Treinen seized the role when he was acquired from the Nationals on July 31, and he still has it for now. The Athletics are determined to give him the chance, even though they have a more experienced option. Treinen has a 1.50 ERA and a 1.25 WHIP.

How secure is he? Treinen wasn't getting much work early, but he's been busier lately, and pitching to mixed results. He's blown two saves in five tries, but he gave Oakland more than one inning in each of his three saves.
Who's next in line? Santiago Casilla is the most obvious option. He closed out the A's 3-2 win over the Rangers on Tuesday after Treinen was forced to exit after taking a shot off his right shin. Casilla has 144 career saves, most of them with the Giants, and was a part of three World Series-winning bullpens.
Mariners
Who's the closer? Diaz
How is it working out? The hard-throwing 24-year-old has been outstanding in the early going, recording an MLB-leading 10 saves in his first 12 appearances while allowing just two hits and one run with 23 strikeouts in 12 1/3 innings. While some of the Mariners' relievers have struggled, Diaz has been lights out in the closer role, and is now the fifth-fastest pitcher in history to reach 200 strikeouts in a career, hitting that milestone Wednesday in his 130th inning.

How secure is he? Outside of a brief stretch early in the 2017 season when Diaz struggled to find his command, he has been Seattle's closer since shortly after being promoted to the Majors in June 2016. There's no reason to think that will change, as he has added strength and maturity to his upper-90s mph fastball and nasty slider combination.
Who's next in line? Veteran Juan Nicasio was signed as a free agent to help fill the setup role, and he has been an effective eighth-inning bridge to Diaz. Nicasio saved six games for the Pirates and Cardinals last year, and he could fill in if needed.
Rangers
Who's the closer? Keone Kela
How is it working out? Kela has only had four save chances, but he's 4-for-4. He has not allowed a run in seven of eight outings. Kela picked up two wins in relief, because manager Jeff Banister brought him into tie games against the Astros rather than waiting for a save situation.

How secure is he? The Rangers have changed closers during the season in each of the past four years, so they aren't hesitant to do so. But Kela has the temperament and the talent to be the closer, and Texas wants to give him a real chance to show he can do the job.
Who's next in line? Kevin Jepsen is the Rangers' backup closer. The wild card is Timothy Lincecum, who can't be activated off the 60-day disabled list until May 28. He is a possible closer, although Lincecum hasn't pitched in the big leagues since 2016, and he's been almost exclusively a starter throughout his career.